Your preamp probably functions as designed. My Counterpoint preamp behaves similarly at low volumes. Here's some info from the FAQ section at the Alta Vista Audio web site regarding this "feature" of volume pots (and high gain tube preamps).
Q: When I turn the volume control down on my preamplifier, one channel goes off before the other. What is wrong and can it be fixed?
A: Continuous-track (non-stepped) potentiometers cannot track perfectly. At some low level, one channel will always go to zero (fully off) before the other. The manufacturers of the parts will tell you this. No manufacturer specifies low level tracking to any degree of accuracy below -50 or -60dB. It's just too hard to manufacture parts that track well at those low levels. Some, however, are better than others. The TKD parts that I use for upgrading Counterpoint gear sound much better than the Noble 25mm parts used in most Counterpoint gear, but track even less well. The tracking of stepped resistor attenuators is essentially perfectly, but the limited number of steps and the large physical size of the parts make them impractical for most Counterpoint preamps. If this mis-tracking at low levels is a real problem for you, you might want to locate another potentiometer and have a technician install it, but it might take a lot of work to find one that tracks nearly perfectly. If you do find a source of good-sounding volume pots that track well, I'd appreciate hearing about it.
Q: When I turn the volume control down on my preamplifier, one channel goes off before the other. What is wrong and can it be fixed?
A: Continuous-track (non-stepped) potentiometers cannot track perfectly. At some low level, one channel will always go to zero (fully off) before the other. The manufacturers of the parts will tell you this. No manufacturer specifies low level tracking to any degree of accuracy below -50 or -60dB. It's just too hard to manufacture parts that track well at those low levels. Some, however, are better than others. The TKD parts that I use for upgrading Counterpoint gear sound much better than the Noble 25mm parts used in most Counterpoint gear, but track even less well. The tracking of stepped resistor attenuators is essentially perfectly, but the limited number of steps and the large physical size of the parts make them impractical for most Counterpoint preamps. If this mis-tracking at low levels is a real problem for you, you might want to locate another potentiometer and have a technician install it, but it might take a lot of work to find one that tracks nearly perfectly. If you do find a source of good-sounding volume pots that track well, I'd appreciate hearing about it.