One of them now has serious distortion on the output. I reflowed all solder joints and cleaned the tube sockets - that's all I did.
Anyway, output of first amp cascade looks good, goes to input is triode phase splitter. Here is where it gets weird. Pin 3 output looks great, whereas pin 1 output of tube is all messed up - bottom of the sine wave is distorted significantly. Swapping the tube doesn't help - output of pin 1 is weird and different with another good 6AN8 tube.
Tube socket looks good visually. I desoldered it, all 9 pins have good continuity through the socket body. I don't have any spare new tube sockets to try unfortunately.
What could be causing this problem? Thank you.