Stereo image moves right to left .

As I lower the volume on my integrated amp from listening levels to "background" level, the sound location moves right to left. That is, if Frank is singing "My Way" in the center of the sound stage at listening levels, he moves toward (not completely to) the left speaker as I lower the volume. This happens in standard operation with the integrated amp, and I recently discovered that it continues to happen when I use the pre-amp section as a pre-amp "out" for a new power amp. This of course would suggest that it is a pre-amp problem, but I am pretty much a "dummy" when it comes to higher level electronics. Is this normal? Is it a pre-amp problem? Repairable? Any thoughts would be appreciated. Thanks.