Some devices feed a lot of noise back into the power grid - a dedicated line won't keep this from happening, as you have discovered. You may need a power filter for you preamp and/or amp. If it is coming in through the ground, one thing that may help is to put some diodes on the ground line. For the 60Hz ground hum I had, I built a plug box and put 3 pairs of 6A diodes - 3 in each direction. I am not sure how/why this works, but it does.
