What pedal are you using Gary? I'm assuming you're not just cranking the amp to get a heavy rock sound? :?
(When you were buying the amp, we were looking mainly at the quality of the cleans, hence the 5F6. Clearly, there are better amps for heavy rock, but the Bassman should be a fine pedal platform for any genre.)
Your speaker pairing of G12T-75 and V30 immediately says "harsh top end except when well cranked" to me, and certainly not excessive bass, as has been said. Neither are my favourite speakers, and certainly not for blues/classic rock. The G12M would be better, especially in view of the slightly scooped Fendery amp.
If you are winding the amp up, turn the bass right down (to zero)as it seems you are doing, but note that if you're using 'buckers with a bassy pedal, this might not be enough. There are loads of mods you can do to the amp (as I think we talked about at the time), from cutting the bass a little, up to the point where it becomes a Marshall Plexi or 2204!
I'm afraid though that you can't have something for nothing. Those sweet, warm cleans, and bluesy breakup will suffer if you want a harder, tighter bottom end.
