Playing through various amps over the years, I've pickup up a couple rules for various models. Basically these are just things you don't want to do unles you want it to sound like crap. Such as
5150s: thou shalt not move the mid knob below six. Even if you're trying to scoop it, boost thy highs and lows.
8100: thou shalt not move the treble above 4, lest you shall achieve the kvltest of toanz.
Rectifier: do not move the bass above 10 o'clock. Ever.