its great how threads go off topic so fast.. this one took a nice change in direction... anyway tho, back to the sub question... are you sure it has to be less than 6 inches mounting depth? that sort of limits your options a lot.. my magnets on my subs are nearly 4.5 inches.. let alone adding the basket, etc. and you wont get to much quality for only 75, 100 bucks.. actually.. you might be able to find a JL10WO or W3v2 for 100.. they're good subs.. theres better (if you want something to thump, id recommend the 10w7.. or even w6s).
some other people will add on for different subs, etc.. alpines, kenwoods, rockford fosgates, etc.. i havent worked with much other than eclipse, jl audio, alpine, memphis, and sony xploders (ugh.. never buy them) in a while, so i can't really point out which others would fit what you're goin for.
2004 Ford F150 FX4
Mods are here and there...