sorry, I've just seen this thread and can't be a**'d to read it all!
I've got a relatively heavy (ok, very heavy) ashdown 300watt combo with 4x10 speakers, and whenever I'm playing in my band (with the guitarist going through his amp into a 2x12 marshall cab) I always find that my tone is getting swallowed up - just as you do. But when I played a gig last saturday with the guitaist going through a 4x12 marshall cab (same head) I end up cutting right through. Really strange....
But one thing that does help is a) setting the guitarists's bass on his EQ on the amp to zero, then pulling off the knob or (if you've got an ashdown) press the 'deep' button (or if you havn't got an ashdown, whack up the bass.... all the way) and you should find a quite boomy tone. However, if, lke me, you prefer tone over bass, then go for option a). :D:D