Hi Mike,
Good question. You could try swapping over the leads from the turntable to the preamp. If the 'weak' channel switches over you know that the problem is upstream of the PP-3. You could also try giving the MM/MC switch a wiggle - that might cure it. Failing that, my best guess would be a faulty preamp. Once the signal has left the PP-3 it is digital and cannot be attenuated in this way, but you could try making a recording in Windows Sound Recorder, just to be certain that it's not a problem with VinylStudio itself (although I'm sure it isn't). You will find this in Start Menu - Programs - Accessories - Entertainment. You could also try another USB cable and / or port, but again I don't believe this is likely to be the cause. I assume you have tried rebooting - always a good bet

Hope this helps. Please let me know if I can be of further assistance. I'd be interested to know the cause, when you do get to the bottom of it.