Having finally restored my morale sufficiently to attack the bolt with a Dremel (it succumbed quickly enough once I decided to not be a wuss and whack the thing up to full speed), and refinished the back where I put the corner protector screw holes in the wrong place, I now have the task of putting the speaker in again. I think one other T-nut didn't want to thread up properly so I shall check them all. This does leave me with the problem of the knackered one, as my endeavours to extract the bolt have basically cut a moat around the hole so I'm not sure that there's anything for a replacement T-nut to bite on at the moment. Would Araldite, the goddess of mending things, work? And if so, smear it over and into the moat and let it cure before putting the T-nut in, or fix the T-nut in place using it?