Okay here is what I've tried:
Plugging the amps in next room via extension cords, didn't change anything.
I did try ground lifters on both amps, as well as on only one; no difference
When I have just one amp(either one) connected to the pre, it still hums. Strangely, with the right amp on, removing left interconnect from left amp increases the hum heard in the right speaker slightly. Conversely, with the left amp on, removing right interconnect from right amp decreases the hum heard in the left speaker. With no interconnects or shorting plugs, there is no problem.
Isomax is a possibility. Would the cheapo similar products(e.g. $12 ground loop eliminators on ebay) give me an idea if Isomax would be effective?
As far as trying amps & pre at a friends' house away from radio tower, that might be worthwhile. Borrowing a different preamp and trying that here will be worth trying too. Thanks to all! Cheers,
Spencer