Help needed to diagnose preamp channel unbalance.

6550c, an Endler is an in-line adjustable attenuator, basically a discrete resistor attenuator. I actually mis-posted there (it was late) and I meant to say an Rothwell, which is an in-line fixed resistor attenuator which some use to adjust (reduce) gain, primarily when CD players have too high an output for a (usually older tubed) preamp. I have a set of Rothwells and also a pair of in-line fixed attenuators I made using Vishay resistors. Both reduce the gain enough to put even low level listening up into the correctly tracking portion of the volume control. However, my fix will be to have it fixed or replaced.

Unfortunately, manufacturers are still using relatively inexpensive Alps pots in expensive preamps because they are the easiest way to implement a remote volume control. IMO a discrete resistor volume control is the way to go, sonically. I am tempted to install a DACT attenuator which uses SMD resistors. Apparently, there are some folks who have figured out how to put a motor on them and set them up for remote, but I do not believe DACT themselves have done that yet.

Alps volume is like "Vishay resistors"
They have different grades, starting at a few bucks each.
Chinese amps have "alps motorized volume pots",
probably made by the same people who make fake gucci handbags.

6550Cs problem is likely the preamp has a grounded cathode and grid-biased using a constant voltage reference. In which case your gain is at the mercy of the gm of your particular tube, so matching is essential.

Counterpoint SA-3.1 and SA-5 are examples of preamps with grid biased with negative power.

So replace the volume control already :) this is a classic volume control-related symptom. Nothing to do with tubes...