I know this is an old thread but I thought to input some information I have discovered on here. I had the same problem as this fellow, I did not like the Mark V at all when I first got it(it had MOMENTS of brilliance allongside hours of well, not really anything worth hearing. I tube rolled (v-1 and v-3 especially powerful for my needs) and finally went out and bought a Triaxis. Lacking a power amp for the Triaxis I slaved my two amp heads to be the power section. Triaxis to a TSL1oo left channel and Mark V right channel, and played this way for awhile (yes I know a Mark V 90 watt as nothing more than a power amp, WHAT SACRILEGE)....HOWEVER using the amp this way SERIOUSLY abused the power section (*really broke it in) and lo and behold all issues gone, The power section on these amps needs some serious breaking in...After all this I went to sell the amp and stuck in the cheapest set of JJ's I had along with the (I thought) somewhat ragged power tubes I had been using and played it to make sure it was up to snuff, and well, to say that it has NOW all the tone I ever could want is an understatement (it almost has too much harmonic content now). These amps serious;ly need some (read ALOT) time to break in properly (BTW I bought this amp used from a touring musician who couldn't get it to sound right for him, so it IS a common problem that can/wll go away). (tweed CAN have enough gain to be a high gain monster if set up right, Channel 3 becomes devastating(I could bnever get the gain levels up to mark iv territory and now it is easy)seriously try this if you do not beleive me (leave your amp on for 24-48 hours just idling and see if it makes any difference, if you think this is bad, Mesa Boogie did this to mine and it helped (it is the first thing they do if they ever take it in is set it to idle for a day or two...if they know it, why shouldn't you!!)
(I would add one PS to everything I said, I use celestion g12t-75's in my marshall 1960A & B cabs, I do not beleive this makes much difference if you know how to use an EQ, but some on here SWEAR by vintage 30's (they also complain of icepick which is nothing I suffer from!!!))