A very easy speaker-protection system you should consider is in-line fuses. I've always used them to protect my drivers after frying an expensive 15" woofer 30 years ago. My Crown amps in fact have line-output fusing built in.

In terms of amps, I believe that a high damping factor also reduces the likelihood of driver damage at any given SPL.

In terms of overall speaker build, my first criterion is the weight of the cabinet. Its really pretty accurate to say that, for numerous reasons, the heavier the cabinet, the better. Of course, this is problematic if you want to hang speakers from walls or ceilings! (I choose not to).

In terms of really bullet-proof speakers, I would have to say the JBL pro line of drivers. A home user would have to work mighty hard to put out a signal that could blow them, and they sound wonderful for heavy rock, but IMHO are insufficiently "fast" or accurate for more subtle music types.

In terms of home-building or kits, I think Jason is over-selling a bit. On the one hand, you will almost certainly have drivers that are technically of higher quality than any finished speakers retailing for under a grand apiece (that's right, apiece not pair), maybe even $1500. On the other hand, true high-end stuff such as Linn (my own favorite), Thiel, etc., is providing a designed / tuned package of crossover / cabinet design and construction that the kits and the home designers rarely equal.

Its curious to me about Def Tech - consistently seems that people either really like their sound or really dislike their sound. Also seems that in general, those whose main interest is HT prefer them compared with people whose main interest is music. I would never call them a "bad" brand or sound, but to my ears they lack clarity in the high end and their bass is overly prominent and somewhat muddy. Having said that, I would take them over anything in the Bose line.