What kind of outlet wall plate do yo use?

I was reading a post over on AA last week. The post was about outlet wall plates, non magnetic stainless steel wall plates.

I had seen these posts in the past on AA but always passed them over never really reading them. This time I took the time to read the post and reponses to it.

To cut to the chase I went out and found some non magnetic stainless steel outlet plates at a lumber yard. I must stop here and tell you I was a bit skeptical that I would hear any difference in sound from my system.

I pulled the plastic plates from my 3 dedicated duplex outlets and installed the ss plates. I plugged the equipment back in and turned on the system. I let it warm up for about a half hour and then sat down for a listen.

The first CD I listened to was Diana Krall, "The Girl in The Other Room." I could hear a difference right off, but not for the better. Sound stage was small, Krall's piano sounded like a blanket had been thrown over it. The air had been let out of the music.
I listened to a few other CDs with the same results.

I then carefully pulled the ss plates from the outlets for the preamp and the power amp. Just sliding them down on the cords out of the way. I sat back down for a listen. Every thing was back as it was before I had installed the ss plates.

I should mention here my preamp is a Sonic Frontiers line-1, and the power amp is an Audio Research VT50.
The three 20 amp branch circuits are installed in EMT conduit. The rough-in boxes are 4"x4" 2 1/8" deep with a single gang raised device cover for the sheetrock wall and electrical outlet mounting. Wire is #10 awg solid THHN copper. Recepts Leviton 20A Hosp Grade. Branch circuit breakers single pole Square D OQ. Branch circuits breakers installed on the same line in the panel. Distance from electrical panel less than 25ft.

I still was puzzled why there was any difference in sound at all. I then, just for the hell of it, installed two leviton stainless steel plates. One for the power amp outlet and the other for the preamp outlet. A magnet will stick to the leviton plates like gorilla glue...

I plugged the gear back in, turned on the system and let it warm up for about a half hour or so. I then sat down for a listen. The sound, not bad. Remember this is the electrical wall outlet plate. First impressions the sound is darker, vocals smoother, more body, fuller.

I will compare the leviton ss plate to the plastic plate later after my ears have had a chance to adjust to this new sound.

Have any of you guys experimented with the wall outlet cover plates?
Why is there a difference in sound? When I first read the post I thought maybe it was due to the rigidity effect the plate was placing on the electrical outlet. Was it working as a damper of sorts. I mean a non magnetic ss cover plate verses a plastic plate.
Why did the non magnetic ss plate degrade the sound of my system?
With the Leviton ss plates are they providing shielding, in effect a Faraday cage.

By the way I asked the person, who posted the thread, why the plate changed the sound on his system. Some of those tweakers can be quite defensive when you question them. Though he was not others were.
I also should mention the member is using solid state gear.

Jim