40hz ratings/speaker response

A lower limit of 40 Hz is not that bad. The trouble is we only speak of fundamental tones and not the harmonics. A lot of instruments have second or third harmonic energy below 40 Hz. If it is a ported alignment, roll off below the 40 Hz FS will be 12 db per octave while a sealed alignment will be at 6 db. Trying to equalize the bass below the FS is tricky since it requires a great amount of power on the part of the amp and the ability to handle it on the part of the woofer. Only try moderate boost of about 4db. That will require over double the amp output than the non boost.

Many would make a good argument that these speaker ratings are largely meaningless. But assuming they are at all meaningful, a rating will include a low end frequency and a tolerance. So it won't just say "40hz", but instead, it will say "40hz +/- 3dB". If you look at a frequency response curve of a speaker, it shows the output in decibels for the range of frequencies at a given signal strength. Ideally, a curve will be completely flat from 0 to infinity, meaning that all frequencies are equally represented. the 40hz +/- 3dB means that the curve is flat within 3db of the vertical center of the curve. In almost all cases, the curve deviates from flat at the low end and you can pretty much assume that that's where the greatest fall off is. So, for a given signal strength, you should hear the music pretty much as recorded, but as you approach the lower frequencies, frequencies of 40hz and less will be 3db or more "quieter" than they should be.

If you at all rely on this spec at all, remember that it is completely meaningless without the tolerance number. For instance, there are many speakers around for under $100 a pair that have a frequency response from 20hz to 30Khz!! (20hz at -18dB, that is!)

A speaker rated down to 40hz will play below the frequency, just rolled off. The amount of rolloff depends on the type of low freq. system. Bass reflex rolls off at twice rate of acoustic suspension below the low frequency resononce. But with room gain, a speaker rated down to 40hz ("faithfully" i.e. +/-2db etc.) would still reproduce down through the 30's and even some of the 20's depending on the room. Small rooms can offer 9db of gain at 20hz. You turn it up, and like with any driver, distortion will go up.

OK, time to get specific. Heres the crux of my problem.
the speaker in question is a B&W Nautilus804. the exact specs are: freq.range= -6db at 30hz and 30khz. and
-3db at 38hz and 22khz. the freq. response rating is:
45hz-20khz +/-2db. Now, to get more specific: last week I dug out an older Bonnie Raitt tape, for one song, just her voice, piano, and an occassional string/cello, lite guitar etc. Feeling somewhat nostalgic I turned it up pretty high(love that piano). Whereupon a woofer started to "rattle". I immediatley turned it down, inspected the speaker, seeing no damage, tried a different cd and the same results with some higher volume. My electronics are Classe CA300 amp, and Classe cp50 pre-amp. My BW dealer has agreed to replace the speaker no problem. But the speaker didn't tear, or pop, or freeze, it "rattled". Could that be something wrong with the electronics and not the speaker?

Wow it's so cool when everyone offers such great insight/advise. Keep in mind that the bass lift most rooms provide is not linear. It's probably your speaker that has failed you. If it's only one channel, reverse speakers to confirm the (1) speaker is the culprit. If not work your way back by reversing the the cable left to right and right to left, one by one from end to source. Eventually you will find the culprit. My guess it will be sooner rather than later. If it effects both channels you will need to find replacement gear and substitute end to source (try changing sources at the preamp first ) or have each piece bench checked. Good luck.