So, I have my favorite tube amp that finally needs some work. I was playing it one day, and noticed that the bass frequencies were cutting out. I fiddled some knobs, and it started acting normal again. It did it a couple times, then I went to flip it into standby mode and it let out a not very pleasant high pitch squeal. Is this just a tube going out that needs replaced? All the power tubes are glowing in similar fashion, not that is always a tell tale sign they are good, but just wondering if this is a preamp tube, or something else? Anyone have this happen, and know the quick fix?