http://forum.sounddomain.com/forum/ultimatebb.cgi?ubb=get_topic;f=11;t=070157 (gonna have to copy and paste link won't work for some reason =/)
Also, should I set the lowpass xover for a 12" idmax at 150hz? It is in a sealed box. The website says the freq response is from 10 to 150 I think. Will it even get that high, and will it sound right? I know most people set it around 80hz.
That's a very good link and correct. The one is use is kind of rusty(the one you posted above), that link is more accurate. He's also right about linear displacement vs. mechanical, we use linear because when you properly power a sub, that's what you're using. SPL competition is totally different. The ID Max will get to 150hz accurately, but won't blend in well with your front components at 150hz. Basically, set the lowpass where you don't notice the subs are in a trunk, this is usually around 80 hz, sometimes 100hz, it just depends on the vehicle and your ears. Lower bass is omnidirectional, midbass (approaching 125-150) you can discern where it's coming from, it'll sound like the bass of the soundstage comes from behind you if the LPF isn't set correctly.