Bloated speakers/weight wise

I guess I look at going from 1/2" to 1" or 3/4" to 1 1/2" MDF a little differently than simply saying it's adding mass. Yes, of course it is adding mass. But more to the point, I think, is that it's changing the ratio of dimensions by doubling the front to back wall thickness (plus it also often entails combining two layers with a an adhesive in between, a form of constrained-layer damping). This is primarily where the extra rigidity comes from. (Think of an I-beam -- it can nearly as rigid as the same dimensions of a solid bar, but is obviously much less massive, which can actually make for a stronger structure for practical purposes.)

And achieving higher rigidity itself is also not always the same thing as reducing resonance. Very non-rigid objects can be the most non-resonant objects -- think of a down pillow -- depending on their form and density. Increasing rigidity raises the resonant frequency (again, all other things being equal, which they likely won't be). Of course higher rigidity in a speaker cabinet is a good thing from a wavelaunch perspective. But without proper attention paid to shapes, sizes, damping, points of contact, etc., higher rigidity in itself might not reduce resonance but only change its character. Overall, to my mind both higher mass and rigidity are probably somewhat overrated compared with self-damping as means to achieve the design goal of a quieter cabinet, and probably more expensive as well, but for the fact that most manufacturers are already set up to deal primarily with MDF.

I thought the Sterophile measurements of the new Mordaunt-Short Performance 6 speaker, that's made of a combination of molded structural foam of varying shapes, thicknesses and densities plus reinforcing metal elements, was fascinating. The result (achieved at the initial cost of a reported 10,000 hours of design time and who what tooling costs) is a 48" tall, 18" deep at the base speaker weighing only 66 lbs., and yielding some very impressive accelerometer results in JA's cabinet resonance tests, with low amplitudes, no dominant frequencies and extremely short die-away. They cost $6,500, not really an outrageous price for an imported, high-tech, nearly full-range 4-driver floorstanding flagship, and the molded-contoured form probably means it has better diffraction properties than most MDF boxes to boot. Of course all that doesn't necessarily mean they sound great, but I do want to hear them if I can.

Vmps RM 40's at 260lbs each. They are not too heavy. I'm not even sure what too heavy for a speaker is. I'd rather have a heavy speaker as they tend to be able to reproduce more life like realism. A smaller speaker usually won't handle as much power or throw as large a soundstage.

I have nothing against a small speaker and if someone never pushes it too hard then it's fine. If you like significant volume and at an at ease sound at these volumes then a larger heavier speaker may be what you need. It's all in your perspective.

I only weight 160 but if there were a flood I'd get those things up the stairs! Moving them around the room is actually very easy on my carpet.

Bartokfan, the front wave from your speaker drivers puts acoustic energy into the room and the rear wave puts an equal amount of energy into the speaker cabinet. Your room is measured in hundreds of cubic feet, but the speaker cabinet is typically measured in the single digit cubic feet. Even at non-loud volume the energy within the speaker cabinet is relatively high and it is this energy that is a major cause of cabinet vibration. The soundwaves bounce around within the cabinet are eventually dissipated, but a certain portion of the sound makes it through the enclosure, or back through the driver and results in a smearing of the original acoustic output. Most attempts to solve this problem involve techniques that will make the speaker heavier. Of course you could take a completely different approach and eliminate the cabinet altogether and let the back wave escape unimpeded into the listening room. This raises another set of problems, but it has been done successfully by several manufacturers.

I see no correlation with speaker weight and the size or quality of a soundstage.

Zaikesman, the M-S speaker has an incredible design for the cabinet. I don't know for a fact, but I suspect the cost of the cabinet relative to the total production cost of the speaker is quite high.

My speakers are 50 inches tall and weigh in around 70 lbs. There is a 10.5 inch fullrange driver at the very top of the column firing forward. They sit on a plinth that is 12 inches square with a spike in each corner. They do not resonate. The box doesn't talk or rock. They play very loud. They are 101 db efficient. They seem to contradict much of what has been written here. I think the stability issue may be due to the fact that my driver cannot be seen to move, even at loud volumes because the excursion is very short. It seems logical to me that less pistonic excursion would be less apt to cause the box to move. As for resonance control, I don't know how they do it but there does not seem to be any cabinet activity at all.
All this leads me to conclude that mass is a substitute (and not necessarily an effective one)for innovative engineering.