Although I don’t think your problem is the power strip, to rule out one more possibility, you might plug them into separate outlets and see what happens. Another thing to consider is the placement of power cords and IC’s. If you can, be sure that they are not close to each other and if they must cross do so as close to crossing at a right angle.
And if you have used the same IC’s with all of your stuff make sure you have IC’s which are properly constructed to reject EMI. I had a right channel hum once that was the result of one IC passing too close to a transformer. (I recently saw a diagram of a Canary IC, LV-77s on Blue Jeans site which shows a high quality insulation/reject of EMI which is used in their own audio cables)
FWIW, there is no easy fix for your problem, I think. Since you have used different amps and preamps with identical results its got to be some where in the electronics preceding the preamp.