MunkeyQ

I've got a little battery powered T-amp which I take along to test speakers/subs out - it's only $30 or so on ebay and it's light enough to take along with an MP3 player. It's only 10w, but that's more than enough juice to work out whether a speaker is fried or not.

Yeahh, this makes me mad too. I once had a pair of very nice ATC home speakers with a big shiny 3 inch dome midrange driver. People for some reason loved to touch the damn thing, and it would dent rather easily. I had to put the grilles on because of it...

Touchy subject this. Anyone familiar with the high-end home audio scene will know it supposedly goes for capacitors in xovers and amps, and interconnects too. IMO, subs do need a certain amount of break in as they are mechanical things. A good designer will design the sub to have the published parameters once the surround and spider have been flexed and softened up a bit. Playing a test tone at a high enough volume to move the cone a bit will break it in.

As the others have said, switch out bits to find the faulty part. I'm betting on a bad amp. The older class D designs had a very simple output filter to get rid of the harmonics kicked out by the switching process. If the capacitance of the caps in the filter changes significantly (heat, age etc.), it's common for a resonance to build up in the filter, causing the problem you describe.