I've been experimenting the last couple of evenings with a bass cut control in one of my guitars. Its taken a bit of playing to get it right, but I thought I'd share my results.
The best so far is to use a 1M pot wired in parallel with a 470pF (0.47nF) capacitor. I've put this in series with the pickups, before the volume control.
I like the effect it has on the Les Paul style guitar I've put it in. When the pot is set to zero ohms, there is no bass cut. When its set to 1M, there is maximum cut.
I tried a 500k pot first, which worked ok, but I saw G&L use a 1M pot so I tried it and it does work better.
For the capacitors I've tried a few values between 4.7nF and the 0.47nF I am using now. The smaller the value, the more the cut. Values around 4.7 to 2.2nF are only really audible on the neck pickup I found.