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Grounding the cover plate.

I'm not sure why your sound is improved when the SS cover plate is ungrounded. Other experience tells me anything we do that increases the antenna extent connected to the safety earth affects the noise in our systems. Most power conditioners suffer from this effect: no matter how much they filter the AC, they still add big metal boxes to the total conductor area attached to the safety earth. Ferromagnetic metal is worse, as it is a preferred place for the magnetic component of airborne RF to go. The rest of your setup apparently protects the safety earth connection to your outlets, so that the cover plate becomes a fairly large additional antenna. You could test this theory by connecting some large metal object to safety earth at your audio system outlet, and listening for degraded sound.

There is some sort of connection between safety earth (chassis) and audio ground in your equipment. Some equipment connects the two at a single point. I've had success in lowering the noise in my system by inserting inductors in these connection points, so that the connection is low-impedance at audio frequencies, and high-impedance at RF. The chassis and casework are still connected firmly to safety earth, but the RF noise on them is isolated from audio ground. This might be a way to reduce the effect in your system if the antenna theory holds.

Another thing to look at is the RF noise produced by your amplifiers' power supplies. This is present in the line cords and more closely coupled to safety earth by any metal objects nearby. Upgraded rectifier diodes, snubbers, filter capacitors across the AC inside the amps, and noise-suppressing power cords are all ways to reduce this contribution.

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