DSL40C's Ultra gain channel stopped working So I'm not entirely familiar with the possible symptoms of a tube amp, so I have no idea what is causing this to happen to my Marshall DSL40C.

The classic gain channel has no apparent problem, but when I switch over to ultra gain, nothing comes out when I play. The only sound I can get is a quick *pop* if I hit the strings particularly hard (as if the signal is just strong enough to get through and make a sound for a fraction of a second).
Also, I've noticed that if I leave the amp on ultra, turn it completely off and back on again, I'll get maybe 5-10 seconds of normal sounding ultra gain sound, but then it cuts off completely and goes back to the state I just described. 

Could this be a sign the tubes a done? If so which ones do I replace? What model do I use as replacement? Where do I buy them?

Thanks for any advice!