Um, I'm no expert, but from everything I have heard and read, going with Fi Qs would be a definite downgrade from your IDMax's. IDMax is regarded as one of the best SQ subs period, so I don't know if you actually do want to downgrade for some reason (???), but if you are looking to upgrade, you need to start looking at stuff like the SI Mag, Aura, DLS, and Focal subwoofers. I don't know which of these come in D2 besides the Mag, but anything else probably won't sound as good as your Max's from what I've heard...
Take this for what it's worth, I haven't heard those subs, just lots of people talking about them. Just my $.02