Too much / too little power "over-drive" speaker?

I know that clipping can occur with too little power, but outside of experimentation, what are the guidelines for having too much power? My system is BW N804's, Classe CA 300 amp, and Classe cp50 pre-amp. Before that I had a smaller amp, and noticed clipping started at higher levels, hence I got the new amp. No Problems at all, very nice clean power, but I'm just worried about getting too much volume, so far I have heard no distortion at relatively high volume, but I haven't pushed it fearing it could blow a speaker. Also, if a speaker does "clip" once or twice, can it sustain permanent damage or does it require sustained clipping? my musical taste is mostly jazz/light rock.
When we say an amp has 500 Watts, we only mean that it is capable of dishing out up to 500 Watts when called for by the source. What's more, volume alone doesn't directly relate to power (Watts). (A bass tone at normal volume may demand more Watts than a piercing high pitch tone). So you can have a zillion watts without worrying about blowing the speakers.

Of course, your volume control can blow a speaker even with a low power amp. However, with your 804's and even most lesser speakers, your ears will usually hurt before you ever get to speaker damage.
Most that know(and I am considering myself one of those for this moment) will say your better of with lots of clean power over your speakers rated "wattage" than too little. If you start overdriving 804's I would imagine the woofers would start bottoming out, and you would hear that. If you are playing the music you say you are, I would have a hard time believing you could do any damage. The speaker can handle huge amounts of power on transients and such, its just sustained volume and abuse, bass control(if you have one) cranked all the way up, distortion being obvious and not doing anything about it.
Personally it sounds as if you have a nice set up, i would think the Classe and the B&W is good sound.
Actually, for what your talking about, you can never really have TOO MUCH power! You won't have to worry about having too much amp to drive those have to worry about having TOO LITTLE amp power is alway's the culprit in my extenive experience. A stronger amp will hang on to the drivers bette at high volumes. But weaker amps will cause the speakers to clip when pushed beyond what the amp can deliver.
My personal experience with the older series Classe amps, is that they're really not that strong! Infact I think the rating they give of doubling power into 4 ohms is bogus! I've blown SEVERAL midrange/tweeters on a few speakers using Classe CA100's/150's/300's, at various times, and notpushing that hard either! Really, If I were you, I'd consider doing a bi-amp situation with those speakers, especially if your playing loud. Otherwise something like Krell 250 or Aragon 8008BB is a minimum to play those hard and loud! The Classe's simply don't have that much power. However, your speakers are modest sensitivity, and I think like 8 ohms(maybe 6). You might be ok, however.
But, in my experience, and I like the Classe sound for the money, don't get me wrong, the classe CA150 is a better sounding amp! You would do much better in selling your CA300, and getting 2 CA150's and biamping your speakers!! For roughly the same money, you would get much much more dynamic sound, and stronger control overll! me. I know, I've done just that in the past with quite a few speakers, when I had both the CA300 and 2 CA150's on hand to do just that! Yes, I know, Ihave too much time on my hands.
good luck
Oh, did I mention that I've personally tried the Nautilus 804's with both single CA150's driving em, and 2 CA150's running biamped!!!...and no was better with 2 CA150's.
Amps control the speaker movement in and out. When an amp clips its because it throws the driver out and doesnt have the power to pull it back in so it looses control of the driver.

The ca 300 is an excellent amp and I would be catious of selling it and getting two ca 150's. In my opinion I beleive quality is the more important then power and the ca 300 has much better parts then the ca 150. I considered 2 ca 200's but ended up with a ca 400 which sounded much better. There is always the option of adding another ca 300 and biamping but if you like the sound and the speakers arent clipping I would think that you would be fine.

You can have too much power but only if you put a huge amp on a cheap speaker. A lot of power could cause excessive travel for the driver and if it had paper cones or cheaper foam it could tear. You have excellent speakers so that is not a concearn.