They basically removed resonances coming off the back walls. As you and I have both noted, Wilsons are often placed in the corners...and reflections in my room really flattened out the soundstage. Just placing them behind on the floor...we have not adjusted for height (yet)...already a lot of reflection-type sound has disappeared so that the original signal comes thru clearer and MORE CONCENTRATED...same for behind sub and also for in front of flatscreen.
I used to compare my audition of Arrakis in Hong Kong as making my X1s feel like Sashas to the XLF...now its more like Alexia vs Maxx3 which frankly is super close and in some rooms possibly nearly identical. I was quite shocked and totally unprepared...then again, I know that going from 0 treatment to 'good treatment' (still far from SOTA treated room) will make a big improvement. And it has.
Again, the signal gets super concentrated so that it is very very easy on your ears with detail, power...more information but easier on your ears at the same time. You have noted vibration under your amps may have weakened the bass...which tightened up with Ultra 5s...imagine that across the full signal spectrum because wall reflections are decreased (without being dampened to dead) and thus the signal concentrates. That's the best description I can come up with for now.
And yes, immediately audible. The best comp was I purposely started with Seal Greatest Hits...sometimes can feel a little like a compressed MP3 file. By the time we were done, we both sat back and realized why Seal is Seal...the power in his voice, the subtle adjustments in his throat as he belts...and it DEFINITELY did not feel compressed or MP3-like anymore...not a Norah Jones album...but in truth, compared with Norah Jones recording...I did not feel the loss at all. Just enjoyed the music. Quite a nice turn of events this weekend.
Big enough improvement, its akin to getting a big speaker upgrade...but for a whole lot less!