I have a Blackstar HT5 tube amp and I like to use a Boss DS1 with the clean channel for the heavy stuff (Zeppelin to Nirvana, these are very different I know). Is this considered a good idea, or should I focus on getting my overdrive channel sounding right and use a Tube Screamer to enhance and boost the tone? This is my first tube amp so this is all new to me.
It really depends what you want tone wise, I have a Blackstar HT studio in my home recording rig with a SD-1 in front, it sounds great, though I do tend to pile that onto the amps gain to get the tone I want.
A tube screamer(Bad Monkey or TS-9), would be equally good if slightly different (slightly smoother with an EQ on the Bad Monkey also).
There are a bunch of other overdrives you could try and I think you really need to try a few out and then decide. I've been meaning to get a hold of the Blackstar Valve Drive pedal, purely because its just the best overdrive tone I have heard in all scenarios. So you may want to try that one out.
As for Lead/Metal tones SD-1,TS-9,Valve Drive, will all do the job for you, and though the SD-1 is a little choppy (i still like it), they are very cheap.
The Idea is that you want to use the amps gain as much as possible to get the natural valve distortion, then just boost the tone a little with an overdrive/screamer of some sort.
All of these pedals produce a lot of noise (Valve drive less than the others), so to get the most out of them, you probably need to invest in a Noise Gate, i use an NS-2 on the floor.
Firstly, read jduv's response. It is comprehensive and provides a view of the options. Your final choices will be subject to your ear and whether you are gigging or recording. For recording, test all the above techniques and try to find other musos that will swap pedals so you have access to many different tones. For gigging, I tend to use a limited set of basic overdive sounds and one very easy way to give yourself a wide set of tone and gain options is to use a EQ (such as GE7) in front of a good valve amp pre-amp. This allows gain boost for solos/heavey disto but also allows you to shape the tone so you can scoop or add mids etc .. (It wil also teach you what you like so you can look for pedals with these characteristics). If you set the crunch channel on a mid to heavy setting you can clean up the sound by backing off the guitar volume. This affects gain to the pre-amp, not the power amp volume. I also use a volume pedal after the pre-amp so I can add more volume for cleanish lead or to regain lost power when my guitar volume is dropped a lot. It also allows you to add high gain (with a pedal?)to the pre-amp but back off the output volume. You can also get very smooth violin affects with the volume pedal here. There is no reason that you can't have a distortion pedal in this set up, but unless it gives you a specific sound, you don't need it.
In order to properly answer this question we need to identify that in the world of guitar effects there are three distinct ways to produce distortion (or clipping): overdrive, boost, and fuzz. Each has it's own unique characteristics. In addition, let's consider the three different ways to run a pedal + amplifier overdrive configuration: clean amp + pedal overdrive, overdriven amp + pedal overdrive, or finally just an overdriven amp. So given these relationships, let's talk about each and hopefully the answer to your question will shake out at the end. In essence, we are building what's called a tone stack.
First, let's quickly discuss the different types of overdrives. I'll tie these in later, so refer back here if you need to.
Fuzz: A fuzz circuit uses an electrical component called a transistor to produce clipping. The ubiquitous example of this is the legendary Dunlop fuzz face popularized by legends such as Jimi Hendrix, Pete Townshend, and David Gilmour. Although there are thousands of fuzz pedals on the market, the Fuzz face is the golden standard. Fuzz pedals tend to have their own personality especially if they are constructed with germanium transistors. They have a very wide range of tones, and blending them with an amplifier is a special case that I'll cover at the end.
Overdrive: Overdrive is a circuit designed to reproduce the distorted sound of a tube amplifier circuit. It does so via clipping which comes in two flavors: symmetrical and asymmetrical. Symmetrical clipping emphasizes odd order harmonics and consequently sounds harsh, while asymmetrical clipping mimics a push-pull amplifier and produces more even ordered harmonics. As a result, asymmetrical clipping will sound smoother, almost as if it were compressed. I go a little more in depth regarding clipping on this question.
Boost: Boost isn't actually distortion, but you use it to get more--note that the following information only applies if you have a tube amplifier or a boost placed directly in front of a JFET based overdrive pedal. Boost pedals come in many flavors, and you can actually use any pedal on your board that has a volume control that exceeds unity gain to achieve the same effect as a dedicated boost. Excluding uniquely built or small wattage push/pull designs a tube amplifier has two stages that we care about for our discussion: a preamplifier stage and a power amp stage. Each of these stages, when overdriven, is capable of producing distortion (via asymmetrical clipping). This is how a tube amp works. As you turn up the gain, it feeds more signal to the pre-amp and power amp stages causing the circuit to overdrive under load and produce distortion. So, couple this with the fact that a boost pedal is designed to take your guitar's signal and amplify it (ideally transparently) before it's fed into the preamp stage and what do you get? More distortion, and more volume. The volume 'goose' can be a problem for some sound engineers, but since boosts are typically utilized for solo applications the volume boost can actually be helpful in getting the solo to cut through the mix. That's another discussion though ;). You can also use boosts to feed more signal into an overdrive pedal--thus producing more clipping there as well.
Now. Armed with this information, let's talk about how to properly blend an amplifiers distortion with a pedal. Not all amplifiers like certain pedals. Darker amplifiers won't take to germainum based fuzz pedals unless a squishy muddy sound is your thing. Alternatively, brighter harsher amplifiers tend to dislike pedals with symmetrical clipping because it amplifies the harshness two fold. Typically there are three different ways to use pedals with an amplifier and each one has it's own set of problems. Let's discuss them.
Just Amplifier Distortion: Probably the simplest route to go is to not use any overdrive pedals at all. In this configuration you simply use the overdrive on your amplifier and augment it with modulation, compression, delay, and over effects. You could use a boost in this pattern to cut through for solos or overdrive your preamp stage past what the dimed gain control would do--which is very useful for getting more oomph out of an amplifier with less gain capabilities. Fender and similar amps come to mind here because they typically use preamp tubes with lower gain ratings such as the 12AU7. A boost could net you a little extra saturation of the preamp or power amp stage in this scenario. A potential drawback of this configuration is that it's locked in to the tone of your amp. Only do this if you are in love with your amplifiers stock tone, or play exceedingly high gain amplifier where an additional overdrive pedal will only add muddiness to the chaos. This pattern is the typical route for metal heads because it's simple, sounds good, and just works.
Clean amplifier pedal distortion: This configuration is where you turn the gain as low as possible on your amplifier and use pedals to get an overdriven sound. The benefit here is that if you have a single channel amplifier you can fake having a clean channel by simply turning off the pedal. Drawbacks of this are that you are stuck with the sound of the pedal you choose--and pedals never sound as warm and saturated as a real tube circuit. Tone models and JFETs can only do so much in the state that they are today. I usually don't recommend going this route unless you want to fake two channels, if for no other reason than you are defeating the fact that you have a tube amplifier. Buy solid state if you want to do this--they get much cleaner and are far better suited for this purpose.
Amplifier distortion and pedal distortion: The combinations are absolutely limitless here, but it takes lots of experimenting to get a tone that works. As we discussed earlier, tube amplifiers clip asymmetrically and emphasize even order harmonics. Pedals can clip either way--so the only way to figure out if something sounds good is to try it. Some general guidelines to follow though, are as follows. Note that YMMV depending on the type of music you play.
As you're going through this process, try to move the gain of the amplifier the least, because it serves as your base. Every time you change the base you'll likely have to completely reconfigure any pedals and boosts you have attached--so it's best to leave it as static as possible.
Finally, let's cover that special case. Fuzz pedals are awesome, but they are incredibly hard to blend with any amplifier or overdrive. Part of this comes from the magnitude and frequency ranges that a fuzz pedal tends to emphasize, and the tone of which will depend on the type and quality of transistors used to build the pedal. One of the major advantages of a fuzz, which should usually be placed very early if not first in your chain in order to sound good, is that they react to the volume control on your guitar much like a tube amp. As you turn down the volume, the fuzz pedal becomes much less saturated. You can use this trick to your advantage to make your fuzz play nice with the other overdrive producing components in your rig. By reducing the input volume to the fuzz pedal you can get a nice "edge of breakup" tone that we talked about in step 1 above without having to use your amplifier. Consequently, you can place your amplifier at a lower gain rating for a cleaner channel and also be able to blend your distortions and boosts with your fuzz. It doesn't work in every case for every amplifier, but that's par for the course. Using this strategy you could potentially get a nice saturated fuzzy tone by turning the volume on your guitar back to full with your entire tone stack (or pieces of it, whatever works best) engaged.
In closing, as with anything in the world of electric guitar gear you should always defer to the pirate codex: "They're not really rules. They're more like guidelines." What I write here might not work for you--but at a minimum you should be able to attack the problem analytically now. Make sure you write all your settings down, take a picture of them, or do something to document it. It's a shame to work for hours to get a tone where you want it only to forget where your settings were and lose it when someone dusts your amplifier + pedal board and moves the knobs on accident (personal experience). Now go experiment, and have fun with it!