I added Grand Prix's Apex footers to my Avantgarde Duo's some time ago, sort of a cone with isolation material and an indent at the 'point' where a big ball bearing couples to a matching receptacle mounted into the speaker frame (the speakers are mounted in a metal frame consisting of three 'columns' and the drivers -the two horns and the subwoofer- attach to the 'sides' of this metal frame). It's been a long time since i first installed them, so I can't tell you now what my memory is of the comarison with the stock feet or spikes. But i do recall 'upgrading' the ball bearing to a nitride (?) one that Alvin offers as an option- I believe it is denser- and I recall that this change, at the time, was noticeable to me and a couple of seasoned audio guys who helped me when i made the change. (I use the same footers/ball bearings under the equipment and amp stands). The floor is a newish, rebuilt wooden floor, covered with carpeting.
Thanks for taking the time, Whart. Good to know. On a 600lb speaker, it is a lot tougher to experiment so guidance is appreciated before i even consider a trial. I have heard Harmonix have a sound...which can be great with the right speaker...N2S less so but some feel the need to let them 'breathe' every few weeks...which i would not wish to do with these speakers!