I'd say the safe bet is to take it to a shop, let them diagnose the problem and give an estimate for repair, if the estimate doesn't line up with what you want to pay, then it's new amp time, the shop probably wouldn't charge too much, and unless the amp is a total basket case they might even take it in partial trade if you buy the new amp from them, I wouldn't hope for too much of discount though. As to what exactly is wrong with it, that's a tough question, but since you have said that it warms up slow and has low power after it warms up I would guess that part of the problem has to be the tube section, have you ever replaced the tube or tubes, I'm guessing it has 1 or 2, 12AX7 preamp tubes, if you have any friends with tube amps they may have a couple of spare 12AX7's that you could try out to see if it helps. I hope this helps, and keep us posted and let us know how things turn out................