Experiencing some intermittent breakup with amp

[syddd]

I have a Blues Junior III and have had a few issues lately. 

At different times I seem to get an unpleasant break-up out of the amp. The most common is when I am stacking pedals (for instance, a bluesbreaker clone into a tubescreamer just sounds terrible for almost all settings), and even having the gain up when having the tubescreamer before the bluesbreaker causes a really unpleasant sound.

My second issue is today I had just a bluesbreaker clone on into the amp, and it seemed to be breaking the amp up in such a way that it sounded unpleasant. Even though I had the same settings used in the morning, and it sounded fine.

Are my ears playing tricks on me, or is there a potential issue with my amp?

If it is a potential amp issue, what would be the likely cause?

[thetragichero]

these come biased HOT from the factory with no room to adjust. a common mod is replacing r51 with a trim pot as seen here: el34world.com/Forum/index.php?topic=19872.0
this assumes that you have a fancy schmancy bias probe. i do not (something about spending a couple hundred bucks on a device good for only one small thing rubs me the wrong way) so i severed the connection between the power tube cathodes and ground and installed a 1 ohm 1% resistor to measure cathode current and deduce bias from that
one of the issues with running the power tubes so hot, along with pcb-mounted sockets and then mounting them "upside down" is that it can cause the solder joints to melt and harden again (poorly) or even lift traces

but backing up, when it is sounding bad, how do the power tubes look? is one glowing cherry red? the one i purchased for cheap used was redplating one of the power tube sockets because the grid stopper resistor on that tube was no good (or the connection to it was no good, somebody else had monkeyed about in there and there were repaired lifted traces, etc) so the control grid wasn't getting negative bias voltage (or signal, for that matter) and it sounded horrible and distorted. take off the back of the amp and tell us what you see when it's sounding bad