Disconnect everything except the 975 and the 7500. Pull all the cables from the 975 and from the wall except for those connections. Fire up the 975 and listen to the radio tuner on a strong station (doesn't matter type of music). Is the hum there or not. If it is, you need to make sure that the 975 and the amp share a common ground at the wall receptacle. Each receptacle has three prongs. One is hot and one is ground and one is neutral. The neutral and ground are connected at the breaker box so they should be the same ground plane. The hum comes from two devices not having a common ground plane and a differential voltage causes the noise. Also make sure there are no stray wire strands touching each other at the speaker connections on the back of the 975 and at the speakers. If the noise is still there try powering the equipment from another receptacle on a different circuit using an extension cord. The problem could be inside the wall if a wiring connection is a little loose or slightly ungrounded.

Now, lets assume that the noise is gone with just the 975/7500 connected. Connect your bluray player via HDMI and listen to it for the noise. If it is there, you need to again find out why the ground plane is different between the BDP and the 975/7500. I have a lot of success plugging all my equipment into the same power source with one common ground.

Do this for your TV and cable box. 9 out of 10 times the ground loop comes from the cable box because it s grounded differently than the house wiring system.

If I read you correctly, when the BDP is connected the TV can see the OSD from the BDP but not the picture from the BDP. Check to make sure the BDP is outputting a signal the TV can see and that the TV is set to receive a 1080P signal. The newer sets are UHD and may need to be set to see a signal less that 4K. I would expect the problem lies between the TV and the Input signal. I would also only use HDMI to start for the obvious reasons.