I had Paradigm Studio/60 speakers on spikes with basement underneath oak floor. I changed spikes to Vibrapods and bass became shorter and more even - I call it more musical. The only drawback of this was lack of mechanical stability (grandson is 5). Now I have Hyperion HPS-938 that comes with spikes (spikes with small plates) and bass is perfectly musical and even (no Vibrapods anymore). In spite of this I'm going (just in case) to stiffen the floor with X braces underneath.
Lossy rubber + construction of vibrapods should give better effect than felt but with resonances you never know. If felt works - leave it. Did you try two layers of felt (2mm is very thin)? What about spikes on granite slab placed on the felt. Spikes should not transfer energy (energy cannot be transfer by point infinitely small) but I found that in reality it only shifts resonances up.
Lossy rubber + construction of vibrapods should give better effect than felt but with resonances you never know. If felt works - leave it. Did you try two layers of felt (2mm is very thin)? What about spikes on granite slab placed on the felt. Spikes should not transfer energy (energy cannot be transfer by point infinitely small) but I found that in reality it only shifts resonances up.