ARC sp10 MkII tube question

Am having some rumble in the left channel. (Have used back up amps, problem doesn't go away - so am assuming it's the pre.) Have checked and tested all tubes and replaced some with ARCs that test strong (am using amperex on the fronts of the phono side.) But the tube in the power supply beside the 6L6s glows white (that fades away almost immediately) when I turn on the pre and all labeling on it is gone. It doesn't look like an Ecc88. Anyone help with identifying it? Any other suggestions?? TIA!
The tube is a 12AT7. The fact that it flashes momentarily when the preamp is switched on does not necessarily mean that it is faulty, but given the apparent age, replacement would be advisable. Please note that a faulty 12AT7 tube would affect both channels, not just one, so that is not the cause of the problem.
Rumble? As in very low frequency? Doesn't sound like a problem I've ever experienced with tubes in the phono stage or the line stage.

Dumb question time, does it rumble when the muted? Does it rumble with all inputs? or just TT. Does it rumble in 'direct' TT mode? Lastly, although I don't why it would matter, have you switched all of the tubes from the left channel to the right channel, including the 6L6's in the power supply?