Channel Imbalance at low volumes

I had the same problem with an NAD receiver, had it in the shop 2 or 3 times, but they never really solved the problem.  In my case, the left channel would be the same volume but only playing the system louder than I typically enjoyed.
Good luck!

Leemaze,
The fact that the volume gain goes up very quickly suggests that input sensitivity is too high, which is something you may want to addres in any case.
I am trying hard to think why the problem persists at the same sound level, but at a different part of mechanical rotation of the volume control when you push the low level button.

I agree that it's likely the volume pot.  I don't know if it's been mentioned, but many volume pots, even in expensive gear (such as my Shindo preamp), just do not adjust volume for both channels equally at the lowest volume settings.  They do fine at medium to high volumes, just not at the lowest volumes.  This is part of the reason people like to have their volume knob at 10-12 o'clock.  What I had to do, since the volume level was high even at the lowest volume setting, was add some 12 db attenuators between the preamp and my amp to allow me to use the preamp volume control between 9 and 12 o'clock.. Balance is fine at that setting, and the volume levels don't drive me out of the room.