How does a speaker blow out?

@mikedc -

                               A test* (if you haven't already):

      *IF your problem/distortion only arises in one channel/woofer: swap your speaker systems to see if it follows the driver.                                   

                                 If not: it's probably the amp.

@mikedc

"Speakers are approximately 15 years old and this has happened before and I have replaced the woofers."

It appears that you may not be getting what you want/need out of the current setup? Woofers typically last 30+ years, so they do not appear to be having a good time with what is being presented to them.

There are numerous remedies here. For one, you might consider a subwoofer to help with the low end. You can accomplish this with a compact unit that does not become an additional piece of furniture in your room to deal with. I wasn’t able to locate specs on your speakers (I tried), but remember the "3db down point" at the extremes translates into half the energy being produced at that frequency. So, say your speakers are -3db @ 45Hz that means at 45Hz you’re getting half the energy that you are getting from the "average" SPL leaving the speaker at other frequencies. So, dialing in a subwoofer @ 45Hz will flatten the response at that frequency and add atleast another octave to the usable bottom end. Also, it’s good to note that if your woofers are trying really hard to pump out the lowest pipe organ notes at, say, 22Hz the woofer is moving twice as far as it does at 45Hz (attempting) to produce the same volume of sound. This doubles the distortion at that frequency and also could "exercise" the woofer well beyond it’s comfort zone.

A little hifi trivia:

Back in the early days of the Bose 901s, they were rated at 270 watts, That’s 30 watt drivers x 9. The problem was that 10wpc receivers were blowing them up. So, Bose revised their minimum power rating to (if memory serves me correctly) 50 wpc.

Once your speaker is repaired you may want to download or purchase a sound level meter. Most music is mixed using 83db (C weighed, slow response)  peaks as a reference. If you are playing louder you may be risking not only your speakers but your hearing as well.

*That should  have read (C weighted fast response)

The maximum power a speaker can handle is an indicator of the full sound spectrum. When the sound is concentrated in only a narrow section of the frequency spectrum, a few watts of electrical power can destroy the driver.