It's only when I flip switches or change the volume, it's probably causing some tiny vibrations that cause the noise. And yes, I use the covers on the 6c45pi's. I'm putting it down to normal HD 800 sensitivity to such things, or to the upgrades the previous owner made.
Anyway, I compared the soundstage of the two amps with my HD 800's and my Norne Vanquish 4-pin XLR cable. Same source, same interconnects, though this is still a highly suspect experiment. The only thing that stands out to me is that the soundstage on the upgraded Mainline might be wider and a bit flatter. It's not deeper, the imaging isn't really much different at all, but the funeral march of my Nabucco recording seems to reach further to the sides on the Mainline. Also, there is no sense of the music being behind me, whereas with the Ragnarok it seems to wrap around a little bit. Also, it should be mentioned that both sound amazing with the HD 800's. Do beware that even though the Mainline is a tubey amp, it's really transparent, it won't help you mask the flaws in a recording.
Edit - Fixed HD 900 to be Hd 800. I don't have the rumored upcoming senn flagship.