Guillaume...i agree with your comments indirectly, since i own Ultra 5s...you have to be very, very careful in my system. I found that using HRS Nimbus Couplers on top of the Ultra 5s...and THEN placing the component on top was 'ideal' in my system. We got the density of tone back which went missing slightly and became 'unlistenable' as a sacrifice i was not prepared to make...and yet i have to admit the clarity and definition that the Ultra 5s brought in instrument separation and bass clarity, depth and detail was ALSO something i was not prepared to sacrifice once i'd heard those benefits.
Since you have HRS...which i also use quite a lot of...M3 Isolation platforms and plenty of HRS double-thickness damping plates...you might just be able to experiment.
My experience with Finite Element Pagode racks and Ceraballs and Cerapucs wasn't too dissimilar. Initially it was all "ooh" and "aah". But ultimately, it was only "ooh" and "aah". I don't mind a bit of "ooh" and "aah", if I'm honest, but not at the expense of flow and touch and nuance.
Too much of a good thing is still too much.
Live and learn.