prevent speakers from blowing

Mr. Whitney, you are confusing THD (total harmonic distortion, a function of an amplifier's component quality) with clipping (over-driving an amplifier beyond it's safe operating limits.) 

It is possible for an amplifier driven into clipping to damage a speaker because it will, in effect, deliver DC voltages to the voice coils.  DC causes a voice coil to stop moving, and since movement is how the coil is cooled, can lead to overheating.  Clipping may or may not damage the speaker depending on the amount of current the amplifier's output stage is capable of delivering.  If the amplifier, even one operating in 100% clipping, cannot deliver more current than the DC rating of the voice coil, then the speaker will just sit there and get warm.  But it won't harm the speaker.  A low powered amplifier can never harm a high-power speaker.

THD will never damage a speaker by itself - unless there is so much distortion in the signal that it is actually causing or masking the fact that the amplifier is clipping.  High THD will simply sound bad.  A poorly built, cheap amplifier with high THD numbers will sound like crap whether it is clipping or not, but it will not damage the speaker unless it is delivering too much power to the voice coil.  

An amp that is capable of delivering more current than the voice coil can handle will overheat the voice coil (or overdrive the speaker, causing it to "bottom out" and stopping voice coil movement) whether it is clean (low THD as well as other types of distortion) or not.  Delivering 20 watts of clipped, noisy signal to a speaker rated for 100 watts RMS is highly unlikely to do anything other than sound bad.  Delivering 200 watts of clean power to the same speaker will eventually destroy it.  That is why, in my main business of designing non-auto sound reinforcement systems, we always allow a 2X to 3X safety factor when matching loudspeakrs to amplifiers.  The speaker must have an RMS rating 2X to 3X the RMS output capability fo the amplifier.

The most dangerous condition for a loudspeaker is being connected to too high powered an amplifier.  The second most dangerous thing is to be connected to a perfectly matched amplifier that is being overdriven and is clipping.  The third most dangerous thing is something sharp being poked through the cone.

The myth of underpower came about largely, I believe, from people buying amplifiers and large subwoofers that are propetly matched, but then not setting them up correctly because they are not satisfied with the sound.  Usually they want more bump than the amp and speaker can safely give, so they overdrive it (usually by mis-use of gain controls) and drive the amp into clipping.  Then when it dies, they are told they "underpowered" the speaker and the myth is born, and it grows and grows...

Geepherder is 100% correct.  Using amplifier "gains" (input level controls) as some sort of power limiter rather than as they were intended is, frankly, uninformed and not effective.

you are correct, i was, indeed, saying THD, when i meant clipped distortion. my error. And i will also concede that an Extremely overpowering amp is a potential hazard, but the original posts "400 w amp to 350 w speaker" scenario is , in my opinion, a safe amount of excess. As for the use of the gains, yes i'm aware that they are intended to compensate for differnt H.U. output voltages (some of the older Kenwood amps had the voltages silkscreened right around the pot.), but once the appropriate level is determined i feel a little  "cutting back" can buy you some safety room. i wasnt trying to imply that it was just a "watt limiter" of some sort.  P.S. i love "dangerous condition" #3, but you should add a fourth "The average do-it-youself" consumer! The most dangerous condition of all....