Originally Posted by JHAz
but those bass guitar speakers are almost universally rolling off very sharply below say about 60 Hz (possibly the vintage era rolls off higher, and for sure it starts compressing far sooner than modern bass cabs (those were guitar speakers with an XMax of a very few mm . . . . ..) So what will happen with a speaker that is actually responding down to 30 or 20 Hz is poorly predicted based on amps sitting on bass guitar speakers. I wouldn't be terribly surprised to hear that even a typical commercial bass guitar cab shook off one of the new era of lightweight bass amps, if it was sitting on a "spongy" stage, played at high levels.
Maybe you missed the part where I referenced frequencies?
X-Max is but one parameter to consider and can be offset by the total number of drivers or the diameter of the driver(s) or both. Volume displacement, Vd, is necessary to create high SPL low frequency output; you have to move large quantities of air and that's where larger X-Max drivers facilitate smaller cabinets and WAF with modern subwoofers. There are other solutions to the same problem. As to the roll off and compression you speak of I can only say that I guess you've never experienced the thunder of a Hammond B-3 playing live in the days before arena based concerts. I've never had the pleasure of playing on a "spongy" stage before so, no comment here.
Originally Posted by JHAz
Frequent, and more knowledgeable than me, contributor Bill Fitzmaurice would, I think, suggest that a decently designed and built sub won't have much vibration in the speaker panels, so from that perspective not a big deal. But it depends on where you put the speaker. On a concrete-on-dirt slab, it'll sit still. In my living room, where the suspended floor is mightily flexible (a hard step in the dining room can be felt at the couch 15 or more feet away), the output of the sub (not it's cab) shakes the floor, which shakes the sub, since the sub is sitting on the floor. Even at my relatively low listening levels, it occasionally bucks off anything lightweight I put on it (like books or CDs etc), but I'd guess a 20 pound or more amp would sit still just fine. I wouldn't put a $10,000 sculpture on top of it, whatever the sculpture's weight, but the amp isn't likely to destroy its own value if it slips a quarter inch off one edge during, ummm, spirited playback.
Sounds like you need to invest in a new floor.
I've a 700 lbs. Yamaha piano setting on my floor and there's no movement of the floor at all.
Back to the OP, I still think you're ok to set your amplifier on the sub cabinet.