Why is balance in my pre inaccurate at low levels?

I have a VTL 5.5, an excellent pre. ONe question: Why, at low volume levels, does the balance shift to the left ie the left channel is much louder than the right? I have to shift the balance knob way over to 3 o'clock to equalize the balance. Is this normal? The sound doesnt seem to be affected.......thanks.....Mark