Generally, Pioneer subs are not highly regarded. I'm not sure what your budget is for subs you just said under $600...If your budget is decent, check out Illusion Audio subs. The mounting depth on them is like 3". They are some of the best SQ subs around. If the budget isn't up to those, take a look at Image Dynamics IDQs, Infinity Perfects, Infinity Kappa, MTX 6000 series, Alpine type S or type R. There are a lot of subs around the $100-$125 range that are pretty comparable. I would go listen to them side by side at a local audio shop if I were you.