Crate V1512 tube recomendations?

A while ago I got my new Crate V1512 15 watt tube amp. It sounds pretty good and works well but I noticed that with the gain maxed and the volume on 12 oclock or above the amp makes a pinging sound. As I edge up the volume a screeching sound starts that getts worse untill becoming totally unmusical and painfull at high levels. I am guessing this means that I have microphonic tubes. What replacement tubes would you reccomend to open up the amps tone. Having a bit less preamp gain would be cool too as I would like a bit more headroom. While I was messing around with that I ran my buddies dunlop hendrix fuzz wah on full into the amp with preamp on full and the volume of half and the tone was so whacked out and insane, a huge wall of whooshing muddy moise, the notes were barely audible.

Part of the problem may be that you have the gain maxxed out- try turning the gain to 12 o'clock and the master up, I have a Crate similar to yours, and I never have my gain past 11 o clock, but the master up around 3 o'clock, sounds great to me.
I have also replaced the first tube with a long plate GT 12at7, because I wanted a little more headroom.