I too have 2905’s. My floor is granite. A few thoughts.
Anything you can do to make the walls, floors, or ceiling a bit rough is helpful. I use a large Persian carpet on one wall, and 12" x 24" rectangular patterns of 1" wood trim on the ceiling.
Equally useful are non-parallel surfaces. You might be able to do something with your ceiling to make it non-parallel to the floor, like a peak in the middle.
Consider your floor; make sure it is as stable as possible. Re-inforcing may be costly, but it is something to consider.
I would absolutely forget about trying to tweek the Quads with power stuff. They use so little current, and anyway the RC time constant is so large, that it can’t make any difference. Further, I tested this theory with high class isolation transformers, and they made no difference whatsoever to the Quads. Big difference to amps, pre, and phono, but nothing to the Quads. Save your money for a better source, or better electronics, or a decent Burgundy.
If you want to improve the Quads, you will have to replace the step-up transformers or replace components in the signal path, like that ugly cap and resistor. Not recommended for a DIY, though - Quads can kill you in two ways. Get a tech to do it. Beware!
Good luck.
