That's tough to say, but it does sound like the sub. If the receiver has preamp outs for the other channels, you could test with one of those (set to large). At least will eliminate the possibility of it being the receiver.
I had something similar with my older Marantz a while back, the surround channels would buzz when played through an external amp. When I switched them back to the receiver directly, they were OK. It did the same on 2 different amps so I knew it was the receiver.