Troubleshooting help please

Are you sure you have a 'mute' circuit? Some which represent themselves as such are nothing more than circuits that reduce volume substantially, like 20db, although the ones I have seen that do are identified that way. If you haven't you might check you manual.

Sorry, I didn't read your post carefully enuf and(mis)applied the info in the 4th sentence to the mute circuit, not the VC. Sounds like a VC issue to me, perhaps in connection with an input gain issue? If you had an input impedence issue it would likely reduce input gain and only in part of the frequency spectrum. Personally, so long as the output is balanced tonally and FR wise, and you have normal use of range of the VC I wouldn't be concerned.

If the gain of your pre-amp is too high you would not have very much use of the VC. Assuming a normal amp input sensitivity, you would probably be restricted to volume control use below 9 o'clock where some VCs may have balance issues and, if stepped, the incremental increases between steps is too high.

For example some pre's have 3db (or more) steps at the bottom of the scale, but when you get up to say 10 to 2 o'clock (the former being probably normal for average systems and the latter for really inefficent systems) the gradient of steps may be reduced to as little as .5bd and gives you a lot of flexibility for fine tuning.

This is sort of generic information, but I hope it helps a bit.