After looking at the schematics, the first suspects for noise would be the electrolytic capacitors in the power supply section, followed by U1 and U4, which are the only opamps directly in the audio chain. They are 4560 opamps and could be replaced with either a 5532 or some other dual opamp that is quieter and has a better slew rate. The other opamps are in the circuit for controlling the VCA so they shouldn't be inducing any noise unless they are having major problems, at which point it won't be working right anyway. Hope that helps. You can download the schematic in Symetrix's tech support area of their website.
I did some more checking and found that not all 501's have 4560 opamps. My blackface one has all socketed LF353, National equivalent of a TL072, while the grey panel has the 4560's soldered in. I actually think the blackface one sounds a bit better.