I owned the IDMAX 12" and had it sealed and thought it was a pretty good sub. I bought a D9 and put it in the SS box that DIamond recommended and LOVE IT!!! I have 1000 watts going to the 10" D9 with no problem what so ever!
I have heard the max's and I would love to have a pair of ID max's. They sound great, loud and smooth. I never heard the d9's, but heard great things about them. I would get the max's over all though. Good luck!