Any amp will sound different at different volume levels. This is partly due to the electronics, but probably more so due to psychoacoustics(the way your ears process and make you perceive sound). This is why a setting that sounds good playing soft in your bedroom most likely won't sound as good, if not completely horribly, when you crank the amp.
I always played with the mids on my 5150 on maximum and very moderate highs and lows...

If you only need two different distorted sounds, use the rhythm channel as your distorted rhythm sound. If you need a clean and a dirty sound, use it the way you described it.