I have been using the IDQ's for a long time and I've got their sound memorized like the back of my hand. I just ordered an XLS10(the car version) from madisound to give it a try. I was going to order the XXLS, but larry said that they are at least 6-8 weeks out becasue peerless is moving it's factory from one location in denmark to another, and it will be at least that long before they are available. He also said that there is a possibility that the posted price will go up by 20% or so due to the costs involved with the move.
He said that they were able to demo the XXLS at CES and that it is nearly identical to the XLS but with a bumped back plate. He recommended that I use the autosound version because it was optimized for much smaller enclosures than the rest of the XLS and XXLS line, and was better suited to situations in car. He also said that with the 300 watts I have on hand, that I would barely be driving the XLS to excursion at 13mm, and the XXLS would be overkill anyways. So the XXLS is definitely a cool woofer, but keep in mind you'll need a bit more power, and it won't sound a whole heck of a lot different. Plus, if you are accustomed to an IDQ's output, the XLS has nearly identical excursion so they probably will be similar in output.
I just plotted the XLS and XXLS autosound 10's and the plot was nearly identical, the XXLS had about 2db's on the XLS from 55 to 30hz. That was the only difference that I saw and probably was the result of the higher excursion.