The DD500b is a pretty mean little guy. The are rated 250wrms and sound great on low power, but in capable hands can handle 500-700 w/ ease. Now, I wouldn't sell them on 5-700w each to an average joe, but if it was in my vehicle I would easily run that much power. Just watch out for unloading issues on that power. We sell quite few of them, been pairing them w/ the DD C2b (~400w@ 4ohm mono, ~600w@2ohm mono). Great for a single or a pair.
I metered a customers little daily ride about 2 weeks ago, G6 coupe w/ 2 DD510b on one of those little Kenwood 500wrms mono amps. Only did 141's, but its very clean. Subs were in 1.4cuft each, 36hz. He was amazed because it beat his buddy w/ 2 CVR12's on the 900wrms kenwood amp in a jeep.
I also threw a little BS box together for a customer to play w/ at a local show. 1 dd510 in 1.3cuft w/ a full length 6" PSP port. Off a 150wrms MTX amp, it did 138's, lol. Not a big #, but fun to mess w/ peoples heads given the low power. He took 1st in his class.
I have also done quite a few 2-DD512 setups in the little AudioEnhancers PB-series prebuilt box. Had a customer doing 144's w/ that in a Mercury Mystique last year. Nice daily sound on the cheap for him.
Also, I have a nice little 4th order they like for small-mid sized SUV's. They get crazy quick and punchy in it, and play LOW.