Epos too have used open stands as opposed to heavy filled ones. I think it shifts vibrational frequencies to different levels but I'm not sure.
What I have found in the past is that filling stands does'nt always improve things. I had some atacamas which I filled and it sapped all the life out of the sounds. Yes, bass was 'deeper and firmer', focus improved a tad but there was no animation to cellos etc anymore. The bounce had vanished. So I emptied them and although I've lost some 'hifi traits' it was a hell of a lot nicer and more natural to listen to.
Incidentally, Onkyo released a speaker based on thin wall guitar panels that resonate with the music just as the instrument does. Boesendorfer also work on a panel resonance principle.
Solid is not always preferable. Just experiment and do what you think sounds best.