Dont do it unless there are provisions for "FRAGILE HANDLING" and as you mentioned "insurance".
I have never transported speakers as check-in baggage but am a pretty frequent flyer using hard case suitcases and a "Sportube" as a case for remote control sailplanes and fishing equipment. The handling that checked-in baggage is subjected to is pretty rough. If you watch the handlers loading and unloading planes you can see suitcases tossed, dropped and generally handled in ways you would not want your speaker handled.
My Sportube looks like this:
http://www.sportube.com/pages/snowboards.html A couple of weeks ago coming back from Florida, one of the handles got torn-up and a line guide on one of the fishing rods got bent pretty bad. I have also had a hardcase suitcase end up with multiple cracks on a corner which I assume happened when it dropped in below freezing weather.
If you have no choice, make sure to double-box surrounding your speaker with "plank foam", bubble wrap, peanuts and hope that the TSA does not decide to inspect it and unravel your packaging.
Good Luck