I made this post on DIYMA but figured it wouldn't hurt to get more data.
I'm aiming more for SQ->SQL rather than SPL
Currently I have a 2006 Pontiac G6 with a monsoon system without onstar. The only thing that isnt stock right now are the front and rear speakers, they have been replaced with Pioneer TS-A1674R 6.5" and Pioneer TS-A1674R 6"x9". (Right now SQ is hurting because the rear speakers do not connect the same way as stock, not sure how to correctly describe this)
I've been doing a ton of research but still haven't been able to solidify a choice as far as what would give me the most improvement in audio according to my budget.