I can’t speak to your technical questions, but I can give you my low-tech experience with my own Rel stacks (3 S 501’s on each side). I set up the stacks and then tuned each by ear, starting with the floor and moving up to the top, one at a time, turning off the other 5, but adjusting the pairs together before moving to the next level. I run them off the High Level outputs.
As to your other questions, I can tell you what I hear, but do not have the expertise in acoustics to explain why or how it works. First the bass is very tight, controlled, and fast, with no boomy, bloated low frequency thumping at all. There is a better sense of space around instruments and vocals, and a far better sense of weight and presence (but not unnaturally up front) with each note. The sense of space is three dimensional, which I think of as the polar opposite of, for example, the Phil Spector Wall of Sound effect. The sound stage is improved in all aspects. I do not notice any nodes or suck-outs, but I imagine that would vary across listening rooms. The overall effect brings the sound much closer to an actual live music than any other component upgrade I have done.