I've been chasing this same intermittent low end resonance / energy build-up issue for awhile, but this isn't really a post about that. Instead, it's a post about a pleasant discovery... mass loading my speakers made some dramatic improvements. I have always had my Shindo 604's sitting on plywood, which sat on my rug. This is how Shindo recommends they be used. After a recent speaker repositioning, I was noticing a lot more vibrations getting transferred to the floor that was causing a resonance (tho not necessarily THE resonance I was chasing). For the first time ever, I removed the plywood altogether and had the speakers sit directly on the rug. The vibrations lessened, but we're still there. I have a bunch of boxes of LP's sitting in an adjacent room, so I stacked 3 full boxes on the top of my speakers. Whoa! The improvement in sound was not subtle. Everything was more solid and focused. I did lose a lot of overall room energy when I removed the plywood (room doesn't feel as pressurized), but that could just mean that I need to reposition the speakers to better load the low end. Or maybe I will try putting the plywood back and then mass loading the speakers. Anyway, I know a lot of speakers aren't shaped in a way where mass loading like this will work, but thought I'd share as I've not seen this discussed much and it's free to try. Ugly, but free. Try it and report back.