The other guys have covered the bases, but I wanted to chime in with some additional points, one very important caution and some other clarifications.
The HT1R sports an ECC82 (12AU7) + ECC83 (12AX7), and for the life of me I cannot find anything that says which tube lives where in the circuit. If I find anything I'll update this answer to be more factual instead of conjectural. Typically in an amplifier of this configuration the 12AU7 would be a reverb tube and the 12AX7 the preamp tube--but even on the HT1 (which has no reverb) that tube combination is listed so I can't even make an educated guess. Most high gain tube based amplifiers have two to four preamp tubes--and all of them are usually the ubiquitous 12AX7--but the HT1R only lists two tubes with very different impedances and gain ratings, which could be responsible for a lack of gain in the circuit.
I absolutely do not recommend that you attempt to toss a 12AX7 into the spot where the 12AU7 lives on the HT1R. The impedance mismatch could cause some serious damage and at a minimum cause the circuit to run out of range and not operate optimally. Since we're dealing with electricity here, when something doesn't run in tolerance you have two options: release any excess energy in the form of heat and burn up or sag--both of which are not ideal. Push-pull amp circuits with preamplifier stages designed for 12AX7's can usually take a lower gain rating preamp tube (such as a 5751 or a 12AU7), but it hardly works right the other way around.
Another thing to consider is that the HT1R doesn't list any power amp tubes. One of the wonderful things about vacuum tubes is that they clip (or distort) when they get too much signal--and lots of amplifier designs have additional power amp stages that you can overdrive with enough signal. My Orange gets exponentially louder as I roll up the volume and boost the input signal with a pedal--a factor of the preamp stage being overloaded and the power amp stage being overloaded. So, the HT1R will not get as loud as a Mesa Boogie triple rectifier which sports a large array of preamp and power tubes for example but since the power stage in the HT1R is likely solid state it can still produce a good bit gain and definitely phenomenal cleans.
Another typical feature on a metal amplifier is the presence of more than one rectifier tube in the circuit (made famous by none other than Mesa Boogie). A rectifier simply converts AC current to DC current--but these magical little components are responsible for the tightened bass tones you hear on many metal records. A reliable rectifier circuit will keep the amplifier from sagging during playing--which results in a tighter, fuller response as the circuit gets the current it needs to run at full capacity quickly. The HT1R looks like it uses a single, solid state rectifier circuit (silicon diode based); that's perfectly fine for you applications--so no complaints there.
The final point I want to make is that the HT1R only has a single 8" speaker. That's hardly enough to push the blinding tight bass tones that lots of metal guru's are known for. Smaller speakers will have a harder time reproducing lower tones (just like in a car stereo) unless you happen to own an amplifier built by Bose ;D. Picking up a closed back 2x12 cabinet that your amplifier can run will definitely help it, but I don't know of many 1 watt amplifiers with the juice to push a pair of 12's at the correct impedance. If you get one, use it at your own risk.
So what can you do?
Well, you can most definitely help your amplifier out a little bit with pedals--but they'll need to be overdrive pedals. A boost will only do so much, and likely push the gain rating on the preamp stage about as far as it will go. Note that you will need to experiment as Faza suggests when mixing overdrives with stock amplifier distortion unless you are always playing the pedals clean. I don't really recommend doing that though because it will really kill the usefulness of the drive stage on the amp and whack most of the tube amp warmth that most of us are in love with. Most distortions sound very sterile unless they are tube based, germanium fuzzes, or hardcore algorithms on a complex modeling unit like AxeFX. There are definitely some awesome JFET based units out there that will fool you though--but the list is very short in my experience.
I have gone through countless overdrives (I recommend you buy used :D) and finally settled on a two overdrive + one boost configuration that I fell completely in love with, so it may take you some time. Be patient. Keep experimenting, and keep playing. My amp doesn't gain out that much either--it's a dual 12AX7 + 4xEL84 configuration, but after discovering a pattern of pedals that I liked and grabbing a closed-back* 2x12 speaker cabinet I can sound like most vintage and some modern metal gurus.
I'm not trying to be a downer here and say you bought a bad amp--but I definitely think that you should understand the shortcomings of the amplifier you purchased versus what you want it to do. Based on my one hour plus of research on the little guy it sounds like you have a really solid practice amp that will last you for a good while--but if you really want to sound like a metal god then you're going to have to pony up and get something louder, larger, and with a power amp stage :D.
*A closed back cabinet is also helpful in recovering some tightness and bass response