Nice looking rig, Dan. The first thing I'd suggest is to space the Crowns out more because the owner's manual for the XLS says to keep things away from the top and bottom because that's where there the magnetic fields are the strongest with these amps.\n\nThe second thing I'd do is try to figure out the frequency of the hum\/noise. Download some pure tones and see which one is closest to what you've been hearing (think of it as an A-B test). If it's ~50-70 Hz, you're likely facing a ground loop issue.\n\nIf this is the case, do not, under any circumstances, change the grounding plug\/pin on your gear. Doing so increases the risk of electrocution on gear that isn't double insulated. \n\nYou can play around with how your gear is plugged in using its original cords. Try to get everything on the same circuit... or same socket, if that's feasible\/safe.\n\nIf this is not the case, tell us what frequency is the closest. This could tell us whether the hum\/noise is EMI\/RF and where it might be getting picked up by the audio chain (such as if speaker wires are even fractions of a specific wavelength up to 1 lambda, which would make them really good antennae).