I'd say replace all the tubes. Bob at eurotubes.com is a big Peavey buff, so I'm sure he can help you out with your amp. He can probably hook you up with some higher gain preamp tubes that will give you a little more boost too. He only deals JJ tubes, which isn't a bad thing because Peavey amps love them.

I'm not sure how much you know about tube amps, but here are a few things to keep in mind. When changing out the power tubes, you need to change them all out at one time with a matched set, and not just one at a time. The bias will also need to be set, unless it's a non adjustable bias (I'm sure Bob can tell you). It's a good idea to let the amp warm up for a few minutes in standby before playing it, and put it on standby for a minute or two before turning it off. This will help prolong the life of the tubes. Tube amps need to be cranked up to sound their best. Single channel tube amps are more versatile than you think. All you need is an overdrive pedal, and you've basically got a 3 channel amp. Crank the amp so you get some nice crunch to it (crunch channel), roll the volume down a bit on your guitar to clean it up a bit (clean channel), and kick on the overdrive pedal for the heavier stuff (lead/distortion channel).

It's possible you could do this with your GT-8. Set the amp to where you get a little break up when you really dig down on your guitar. Set one channel of the GT-8 to have no effects whatsoever, no amp model, no cab, no nothing (this will be your clean channel). Then set the next channel just like the previous one, but only turn on one of the overdrive/distortion pedal models. This will be your dirty channel. This wont sound as good as using an actuall pedal, but it might work for your needs. But, whatever you do, don't use any of the amp or cabinet models in the GT-8 with your amp.

I would defanately get some new tubes for the amp before playing it too much more. You don't want to damage it. Playing with a failed tube could cause damage.

I hope some of that helps. ;)