My stiletto seems to be acting up lately.I notice it mostly on channel 2. It sounds rather thin and tinny almost like a wah pedal is turned on. Its almost like the mid and bass are not coming all the way through. It still gets loud but doesnt seem to lost its thump. I have checked all the tubes and they are glowing. Tried switching from spongy to bold, silicone to rectifier and 50 to 100 watt. I swapped the pre amp tubes in v1 and v5 but still no change. Any ideas?? is it possible that I have some bad power amp tubes that are still glowing??