Speakers are probably 10 years old. It really is just the woofers, not the tweeter. I'll try to turn the bass down completely and see. Wouldn't I notice buzzing at virtually all volume levels if the woofers were blown?
Thanks all for your advice. I did check back with the person who sold them to me, who seemed quite trustworthy. He says the speakers weren't blown. I do know that I have to turn up my amp quite a bit higher to get the same sound level as with my earlier speakers, so I'm thinking indeed it is the sound of clipping. I'll try to verify it by using the speakers with a different, more powerful amp whenever I get the chance.