Cheap tweak from Virtual Dynamics:

We've got too much time on our hands! Brass mounting screws vs. steel, don't you wonder how much affect this can really have when their mass is so insignificant relative to their proximity or lack thereof to the voice coils magnetic field? I wish my brain could retrieve this tidbit in totality from memory, but I remember a speaker manufacturer speaking about the issue of screws, speaker mounting, and mdf. It may have been the owner of Magico in a recent Stereophile or Absolute Sound issue. Anyway... he talked about the dangers and deficiencies of using mdf as cabinet material because of it's propensity for losing ability to lock screw threads with the stresses of constant vibration and the passage of time. No doubt part of his spiel is self serving, but as someone with a bit of experience fabricating sculpture and furniture, I can attest to the potential for weakened threads if you back out and replace screws in mdf. Personally, I think the dangers likely outweigh the potential rewards. At the very least, I'd think about putting the speaker on it's back side after removing the screws and reinforcing the thread walls with tiny amounts of thin superglue applied with a needle applicator.

Tom, yes I've thought about aspect that since I posted originally. I'm still amazed that screws alone could have a big impact on sound in a typical speaker mounting situation. The low mechanical impedance of brass is still constrained by the gasket material around a speaker basket, the vibratory characteristics of the speaker basket, and the vibration damping capacities in the many materials used in a speaker. I wonder how much relevance can be drawn from the musical instrument analogy. An instrument is designed to vibrate and radiate sound, no surprise increasing mechanical coupling to the floor enhances sound. A speaker, to greater or lesser degree depending on manufacturing budget and skill of designer, is supposed to be as inert as possible. Of course we know from looking at all of the accelerometer graphs in Stereophile that all cabinets vibrate to greater or lesser degree, so the coupling angle you mention may well be a factor. This all got me wondering what effect might be had from using brass or bronze as speaker basket material instead of the usual suspects.