Speaker hum

With respect to the hum you are hearing now, all the speakers would have done is make whatever was already there 8 dB more noticeable.

Is your set up attached to a cable box, i.e., you play your tv through it occasionally?  This is a commo source of hum, which I have experienced.  Needed to simply purchase a Jensen VRD-1FF IsoMax Digital RF Isolator which handily took care of the problem.

Very easily, that is a huge jump !

With speakers that efficient and an amp with a ton of gain, you’re likely hear the slightest hint of residual noise, coming from anywhere in your system. Why did you mate speakers that sensitive, with an Odyssey Stratos amp?

I discovered a light dimmer in my room was the culprit.

Make sure your TV cable coax is not the culprit if using in an AV system. 

I cleaned up everything behind my rack and made sure the power and source cables don't intersect at all.  Unplugged everything from the preamp and when I remove just one channel between the preamp and the amp it stops. It seem a little louder now and you can hear it at a 4 or 5 feet.

I have a cable hooked to the cable box to the AV receiver that connects to the preamp for using the mains for the front channels in a HT bypass in a 5.2.

The outlets feeding the system are straight from the breaker panel, nothing else on the circuit, but I have 3 different light switches with dimmers for LED lights in the room on their own circuit.

I didn't know that I was shooting myself in the foot by pairing HE speakers with the Odyssey amp.

I have experienced hum from dimmers even when speaker cables were disconnected at the amp and the conductors twisted together to simulate the output impedance of the amp,  close to 0 ohms.  Fixed it by installing twisted pairs for the cables similar to professional speaker cables from Belden, Mohawk and others which reject common mode noise.

+8 db is slightly less than 'double' from a hearing perspective. Likely cause is difference in speaker and room response if that is the sole change.

Why are the grounds commoned in the wall box for two circuits? If there are two circuits, the ground should ONLY tie at the buss panel bar.

I'm assuming two AC circuits and one amp on each.

Are you using balanced cables? If not, try them. The screen should only be connected at the pre amp end.

Start with just amp with back shorted inputs connected to speakers. If it is quiet, connect only preamp with nothing else. If still quiet, add one piece at a time until the noise appears, then deal with it.

Sadly Odyssey does not list amp gain. It would be possible to pad the amp input if dropping the gain 8db removed the noise, which I doubt.

I built studios in Los Angeles. I also fixed botched installs. ALL could play on STUN without any noise whatsoever. PM me if you wish to avoid the i......t, [e.g. DIMMERs don't hum, they buzz!] chatter here.