Yeah, those woofers really load those cabients even though they are open Did you line your cabinets with no-rez ? Until I really crank up the SPL, I can place a looney on it's edge on top of an h-frame and it will sit like that until it's pretty loud.
I don't remember how thick you made the top plates but I've found that using 1" as opposed to 1.25" - 1.5" makes them a bit more suseptible to vibration, the no rez does help.
I'm using some Sorbathane under a pair of MTM's right now, may give the Herbie's fat dots a try at some point
jay