Overdriving speakers, or clipping?

Standard disclaimer applies: this was only a test (but the result has me curious).

I was driving a pair of Boston Acoustics A60 bookshelves (89db, 8ohm, 75w, 1" tweeter, 8" woofer - big for a a bookshelf) with a B&K ST1400II (125w), giving a new preamp the flight test. I turned it up about half crank and the sound started getting harsh, and cutting out. I quickly backed it off, and grabbed another pair of speakers:

I took a huge pair of Wharfedale towers (valdus 500, 93db, 8ohm, 200w, 3 x 8" and 28mm tweeter) out of the basement, and played these very loud. They went MUCH louder than the point at which the BAs were cutting out, with no problems. Sounded pretty good for cheap speakers, too.

I've heard amps clip when they run out of juice, but it's hard to imagine the amp ran out of juice pushing the efficient, smaller BA's. It must be something else causing this - like overloading the crossover or internal wiring of the speaker, and causing some other effect (or perhaps the amp IS clipping as a result of something happening within the BA when it got too much power).

Any explanations for this phenomenon? I've never overdriven a speaker before and produced audible dropouts - I've heard plenty of speakers distort and lose their cool, but this is a new one for me ; )
You may have been running into problems on both ends. That is, the amp was starting to give out and so were the speakers. The fact that everything was fine with another set of more efficient speakers that handle more power somewhat confirms this. I say that because with more drivers and the increased surface area that goes along with that on the Wharfedale's, they can more easily handle the power AND produce higher spl's with less strain. Since they are more efficient to begin with, the amplifier is also not working as hard overall. Bottom line is that the speakers weren't being pushed as hard and the amp wasn't being pushed as hard. The results speak for themselves. Sean
You may have been hearing what happens when an 8 ohm driver dips below 8 ohms and shows the amplifier a 4-5 ohm load (especially at low frequencies). The Wharfdales are 4db more efficient than the Bostons which means they will play more than twice a loud with the same amount of power. Also the Bostons might be a little optimistic about that 89db efficiency.
If the Bostons are very old check the woovfer surrounds, they may be dry-rotted.
Rwwear, very astute - I have already replaced the surrounds (a visiting toddler poked a finger through one of the surrounds, making me realize they were rotten - thanks kiddo, I really mean it!) The improvement in sound is obviously quite significant.

Thanks everyone for the comments - all right on the money.