180 watts and a 15" speaker........... (I've played through one - marginally OK....)

You are going to have to use a fair amount of that 150 watts to make a 12" speaker sound as deep and full as a 15"speaker at even a practice level..........

Remember, you are trying to reproduce everything from Kick Drum to high synth sounds through the same speaker AT THE SAME TIME....... this isn't a guitar you're playing!