High pitched squealing/screeching through amp

I had my M-Pulse at practice a week ago and everynow and then it would start to hum loudly and then start screeching madly. It happened a couple of times and when it started happening one time, I turned down both pickup volumes on my bass...still the sound came and I had to shut the amp off.

The conditions:
*We were playing loudly in flannel shirt weather. I've run the amp very loud in hotter weather.
*Using a speaker cable and 8 ohm cab
*We practice in an old guest house in the backyard...possibly unstable voltage

Any ideas what this could possibly be? It's never happened in my house, but then again I never run it that loud in my house. The vents are always unblocked. Could this possibly be a sign of bad tubes?

Please

 

Ah ok good. This initially sounded like a much worse problem...I didn't know tubes could literally scream "replacement". I've heard bad tubes whistle before, but how often is it you get a screecher?