software

I did some trouble-shooting on my brother-in-law's Yamaha Strat clone the other day; it was working only sometimes. What I found was a couple of poor solder joints; easy fix.
The only problem i hear now is a noticable quot;humquot; when the volume knob is turned down. Not super-loud, but there. Full on; no problem. All the way down; no problem; just anywhere in-between.
He replaced all of the PU's with Carvin's a while back, H-S-S, and in particular the blade-style singles sound not-too-bad. I double-checked the wiring on the Carvin website and it appears that he did everyhing right. It hums the same on every PU, every combination. It's wired with one volume, one master tone, 500K mini-pots. Seem to be fairly hot PU's.
Anyone else run into this problem?