I started using SR's many years ago, at first a single up near ceiling, front wall, centered between speakers. Sound stage greater height but more recessed than without. Added second near ceiling, behind listening position, centered. This pulled sound stage forward and intensified solidity of center image. My SR's had potentiometer which varied intensity of these effects. I later added linear power supplies, thought I may have heard improvement, perhaps placebo effect here.
Over the years I tried various positions of both sr, sound stage always pulled towards position of sr. Near ceiling always best sq.
Over these years I periodically removed and inserted srs, always preferred sr in. However, as my system improved from various equipment changes, therefore, more dimensional imaging, larger sound stage, I found sr's to be detrimental. The pull of sound stage with sr's created a more artificial ss, center image always compressed, especially width. Without sr's, more natural proportion of width, height, depth of ss. I've not used my sr's for perhaps two year.
I never perceived any lowering of noise floor with sr, ss changes only effect I heard.
In thinking about these ss effects just now, I'm curious if four sr's could be superior to none. All centered near ceiling, front wall, behind listening position, two added on side walls. Decrease intensity and widen out center image. Not curious enough to try.
Overall, my experience was initial enthusiasm followed by slowly diminishing positive results. In the end, another failed tweak. Still, I can imagine sr's could be a positive in some systems and rooms, not a bad tweak by any means.