#1 Phase setting is used to get it "in line" with the comps, any dealer should be able to help you with that
#2 From one point subsonic filter is a good thing - it takes a lot of work for the amp to play those very low notes 15-30hz. So you could set the subsonic filter at about 30hz, so your amp fades out below 30hz and it will be easier for it to perform, well, depends on the slope for the subsonic too.
#3 Mainly cheap amps have bass boost option. It's prob at 50hz. If you crank it, it will make your amp really sweat. For +3db setting it will use 2 times more current to play those notes. I would leave it at 0.
It got hot because it's Audiobahn You get what you pay for. I used to have RF400.4 amp and I could cook eggs on it after about 20 minutes as well
