I agree that looking at the future configuration you want is a good idea becuase that way you're not spending money on something now and then spending more to replace it later. From that point of view, one piece of advice is to imagine your ideal, complete PA system that you hope to have in the future. Then, when you want to add or change what you currently have, you buy with the ideal future PA in mind.
Regarding subs in general - Subs bring two things to the table: low end frequency extension, and more power to the system overall. The first is a lot less important than one might think (and also more difficult to attain), the second is extremely helpful. I'm not sure of all the technical or acoustic reasons why, but if you had a 1000 Watt system and you wanted to make it a 2000 Watt system, it's better to subdivide the frequency range and have 1000 Watts for lows and 1000 Watts for mids and highs than it is to just add another 1000 Watts to a full-range setup.
The reason that is relevant to you is because if you don't cut the low frequencies to your powered mains when you introduce a sub(s), you're not getting as much benefit from the power boost that you would normally get from adding subs. The other downside to not putting a high pass on the mains/tops is that you're more likely to have destructive interference of low frequencies between the tops and the subs.
Regarding full-range vs. low pass vs. crossover - It might be a challenge to actually high-pass the mains feed when using a powered mixer (unless you have a main out insert), so for now I wouldn't worry too much about it. It's a good thing to plan in advance for if and when you change out your mixer. To that end, you should consider getting a crossover to low pass your sub feed, which you can then use in the future to high-pass your tops. Either a passive EQ-only crossover like these, or if you want to future-proof your rig a lot more and have lots of options, you can go for a speaker management system like these. The speaker managers cost more and offer delay, EQ, and other processing. The delay processing in particular can come in handy for time-aligning subs with tops, which makes a surprisingly huge difference when you get it right. The downside with speaker managers, besides cost, is that they are more complicated and might be overkill for mixing from the stage if you don't have your own sound person to really tweak it out.
Regarding one or two subs - you can divide that question into placement and budget. You don't need two subs for stereo purposes, partly because you don't want to run a non-stadium PA in stereo anyway (and most stadium-sized PAs are run mono also), and partly because even if you did run stereo, the low frequencies are poorly localized by our ears so there's no benefit to stereo spread for subs. That also means that placement is down to time alignment, coverage, beaming, and visuals. Acoustically, the best place for any number of subs is right in the middle of the stage. A lot of club-level venues build a stage tall enough to put the subs under the stage behind a black scrim. If you don't have a tall stage, then you usually want to put the subs near the tops, which are typically off to the sides so they don't block the view of the band. Putting the subs near the tops also helps with time alignment, as mentioned above regarding delay processing. If you only have one sub, and you can't put it in the middle, then you're stuck with choosing a side to put the sub on and that means people closer to one side of the stage will hear more bass than people on the other side. The downside to separating the subs is there can be destructive interference in the middle that creates hot zones and dead zones in the low end (the problem is often called the power alley), but typically that's a necessary compromise for bar-level gigs. If you can afford it, I would say two subs is preferred because you can place them together in the middle if possible, or spread them with the tops for better coverage and time alignment (and just endure the power alley in the latter case).
Regarding powered vs. unpowered - This is a tougher question that probably only you can answer. With the concept of how you want your whole system to look at some point in the future, and assuming if you go powered subs now you'll go for powered tops later, let's look at some bullets.
Using Powered Mains and Subs
- Overall, a powered system can be more compact. With powered subs and tops available based on 12" drivers, and without the need for a power amp rack on stage, a fairly high-wattage FOH system can fit in a smaller vehicle for bar-sized gigs.
- Running power to powered systems can be very tedious. A roll-up multi outlet extension cord for each side of the stage is usually perfect for this, but that doesn't change the fact that you have to run both power and two signal cables to both stage right and stage left. Since the signal cables for powered speakers are line level, instead of speaker level, they can be more susceptible to noise.
- Manufacturers can tune the amp to the enclosure very closely, creating systems with more ideal damping factors and internal bi-amping (for tops).
Using Passive Subs and Tops With Separate Power Amps
Usually these systems take up more space than systems based on powered speakers, since the cabinets are usually about the same size and then you add on the amp rack, which has to go somewhere. Usually the amp rack goes on or near the stage to take advantage of snake returns and shorter speaker cable runs.
Once you have the amp rack(s) powered up, the only cables that have to run around the stage area are speaker cables, which helps keep stage cabling neater. Also, good quality speaker cables are more durable than line level cables and it's easier to keep the amp rack and power supply out of the way of stray beer spills as opposed to powered speaker power supplies.
Separating the amps from the cabinets can save your back, since you are dividing the weight up into separate boxes. On the other hand, the total weight of the amp rack(s) and cabinets of a passive system will usually be more than the total weight of a powered system. So passive is better for your back(s), worse for your vehicle suspension.
If you blow the driver in an unpowered speaker, you can more easily and cheaply repair or replace it and/or rent a substitute. Likewise any beer spillage, toppling, or other trauma is less likely to cause a complete failure of an unpowered speaker.
You can choose your favorite speakers separately from your favorite power amps, versus a powered system where you might get a great enclosure with a so-so amp or vice versa. Overall I've been more satisfied with the sound from unpowered speakers and separate power amps, and this reason is my favorite theory why.
Purchasing and using unpowered speakers with separate power amps does require more understanding, research, and careful product selection to match power and impedance levels appropriate and also to set crossovers, filters, and limiters appropriately to get the best sound and protection of the speakers. Powered systems can be tuned and configured inside the box(es) for the ideal settings, making them a bit more plug-and-play.
Not totally related note about mixers
I want to put a personal recommendation against powered mixers and in favor of at least considering a digital mixer if and when you look at replacing your current mixer. Powered mixers are nice compact ways to build the smallest systems, but they have so many limitations and you can't upgrade power levels without upgrading the whole mixer. Digital mixers are nice because you get effects and more flexible EQ built in without having to build a separate effects rack, making the whole system more powerful and compact at the same time. These days sound quality is not at all an issue with digital mixers.
