I used to rock one of those amp, pretty great for what they were! I would use the crunch channel, maybe use the fat switch? I never really heard a big difference with that, but when you are using an overdrive in front of the amp, it helps to have a fat tone to begin with so the sound doesnt thin out too much. Definitely play with the level and gain on the pedal a bit. You can definitely get some good results with how Adam mentioned. It really depends on how the amp and pedal interact with each other. I used a boss super overdrive and kept the level a little past the half way point and the gain a notch below it. But yeah as a general rule, I keep the gain a little lower on the amp so it had a bit of crunch but isnt not super saturated, dial in more mids and bass to make things fuller, and then throw the overdrive in front of it to tighten things up a little bit.
Also, that amp can be a little squeely at times, especially if you add an overdrive. If I remember correctly, you can turn the send and return volumes down on the effects loop to maybe 2 or 3, engage the effects loops(and keep it engaged), and then you can turn the master volume up a bit without it getting ear piercingly loud.
If you go a little further to get that metal tone you desire, I would get a noise gate to clean up the unwanted noise and make it so NOTHING is coming out of your amp when you arent playing. I HIGHLY recommend the decimator noise gate, it’s nearly the perfect noise gate and it only has 1 knob so you dont have to mess with a bunch of settings and find a balance.