Channel Imbalance at low volumes

You may want to get an audiogram, to rule out a hearing imbalance. Recruitment occurs at higher volume levels.

Or use headphones and see if the problem still exists

Only listen at higher volumes... problem solved.

Hello from Costa Rica. 
Your problem is the potentiometer of the volume control.   At low level, the left channel, due to its age, had became more resistive than the other channel.  You must change the pot, and that's it.  You can measure the resistance at any point, the left channel should be equal to the right channel or very close.  Usually the tracking balance is +-1.5dB between channels when it is correct for the type of integrated amplifier that you have. 
If you want to do an updated, you could use a step attenuator:


Happy listening!

No traditional pot meter is particularly accurate at low levels. Does it happen with all sources? I ask because there may be a mismatch between the output sensitivity of the source and the input sensitivity of the amplifier. Many modern sources like disc players or DACs have the nominal 2.0V output of the CD Red Book standard, but some have even more (to impress in the demo room - louder sounds better). At the same time traditional rca inputs are far more sensitive. This creates two problems. The first is that, depending on the amplifier topology (where is the volume control, before or after the input stage?) the input stage may be clipping from the high signal (with all the sonic issues that come with that). The second problem is that if you want to play at a comfortably low level, you have to turn the volume control down into the range where the pot meter is not very accurate, and you get channel imbalance. Do you get a pretty loud signal already very quickly, i.e. when you turn up the volume control perhaps only a quarter? If that is the case, you need to reduce the input signal. Inline attenuators are the simple solution.
This is not to say that the pot meter may not be failing as well. But even if it is no longer at its best, inline attenautors (or an attenuating cable) may still be a cheap fix (Partsexpress has attenuators from Harrison Labs).

@willemj it does indeed happen with all sources. The volume does gain VERY quickly, however the amp has a built in “low-level” button which brings the volume down by 4-5dbs so the knob can be turned up more. Would this be done by a built in attenuator? However the imbalance is still present at the same low audible levels. 

I tried lots of deoxit however it didn’t help. I couldn’t really get to the pot though so I was shooting a little blind - is there any source for a schematic or service manual?

In addition, I moved the amp to my kitchen which is a large room (~40’ long) so I’m listening at higher volumes more often and it’s noticeable less often. 😉