The IDMax is a special subwoofer. It doesn't need a lot of power to get stupid loud and it sound phenomenal. If you can fit it (it's deep) I think you'll love it as it will get downright violent with 500w. A lot of people like the W6v2, myself included, but I actually like the older W6's & W3's better. I've owned both new & old school W6's btw.
I looked into picking up some Fi woofers (Q's or SSD's) and I wasn't really thrilled by the amount of power they require and the email tag you have to play with them to get vague answer. The prices seem quite high too for a woofer that really doesn't seem that special. It's really deep, needs a bit of power, and it didn't suit my application well.
Someone mentioned Alumapro; those things are awesome. Are they still around? I had a pair of IB 10's that I absolutely adored...should've kept 'em. Their other subs are no slouches either. They're all SQ oriented yet are so efficient they absolutely pound with a small amp. No need for electrical upgrades and you're not sacrificing SQ or SPL. This kinda goes along with most light-coned woofers out there which Image Dynamics and JL Audio would both be in that category.