The advantage of the crossovers, is that the mids / highs are allowed to reproduce higher frenquencies so they dont distort, and allow the subs to play the low end. The alpines I mentioned above do exactly that. A 6x8 would make some nice midbass, esp the Inifinity Kappa or the higher end Pioneer 6x8's. If I had doors like that Id put 8" mids in myself (but then again I am an audio nut)