BTW, I don't think I made it clear in my post above that I never heard any buzzing or rattling while music was playing, but the sound still suffered.
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Thanks for the post Alrau. You are right in pointing out that secondary, sympathetic vibrations from other parts of the listening room can muddy the sound. That's why it's important to have very stiff surfaces or non-resonating absorbers (e.g., curtains, cushions, sandbags) in the room. I'd wager the floorboard was acting as a third speaker, but out of phase and with poor resolution -- it was resonating with the music.
The floor was probably not contributing as much as you'd think. When I had the addition with the music room put onto our house, I specified that they put the floor joists on 10" centers rather than 16" centers (and I think they sized the joists one size larger than code), with carpet and padding over the 2 layers of 1/2" plywood. I also have about 200 pounds total of weights in various places between the speakers (plus a 36" direct view TV). All that weight did improve the bass significantly; I discovered this when I chucked out a 40" projection TV that weighed 195 lbs. and noticed all the bass had disappeared. Borrowed some weights from my neighbor, and, voila! Bass again.
While I'm sure the floor must "sing" to some extent, it doesn't obscure detail. The baseboard did.
Alrau, I meant to say the baseboard rather than the floorboard, but either could wreck things as Jayctoy attests. Large solid surfaces that resonate with the music can wreck havoc.
I heard a Bosendorfer speaker not too long ago during a private audition. It had a solid wood side that was allowed to move and was designed to act as the woofer. It worked (though quite honestly, I've heard better low end in most other speakers). Moving wood does act as a speaker.
Alrau, good thinking to add the joists. I redesigned my flooring support to overkill the loading as well. I'm glad things are working out for you now.
I'm a heating contractor and familiar with "buzzing" baseboard; there are a couple of alternatives available. One would be switching to cast iron baseboard, which is very non-resonant, but also much more expensive than regular copper and tin b/board. The other alternative would be to add under floor heat to the room and eliminate the baseboard altogether. There are relatively inexpensive ways to do this, using your current boiler. If you have access to the floor joists under that room it is very simple; you can utilize your current 180 degree (+/-) boiler water without getting a hotfoot. A good hydronic heating contractor in your area could help you out, and much nicer heat, too.
I have the baseboard problem pretty much under control now(and my system has been absolutely transformed). The original heat in this place was radiant heat using copper in the cement slab. It broke and was replaced with electric b'boards before I moved here. I installed a kerosene furnace when we moved in (heated 1200 sq. ft for $300 or so a year), but ultimately we put in hot water b'boards throughout when the addition went up. And you're right--radiant heat is nice. My wife loved putting her feet on warm flooring!