Slayerx
10+ year member
CarAudio.com Elite
Depends on application and many variables on which is "better", which will always just be an opinion. Both are great "sql" subs, but I think I'd go ID for a sq setup and Fi for a more spl oriented sub. I've heard both subs demo'd, and I think the ID was more natural sounding then the FiQ I heard. But then again, different setups in different vehicles yeild different results.
Overall though, if I were purchasing either one, I think I'd feel safest choosing t he IDMax
Overall though, if I were purchasing either one, I think I'd feel safest choosing t he IDMax