sh!tbynines won't produce much bass when put into a small box like that. Most all have mechanical qualities that make them suited for large infinite baffle setups. ie. rear deck of car. your better off running small round subs in a proper sized box. round speakers = less distortion. Very true about the rear speakers.. it destroys the sound stage. there is no good reason to have rear speakers playing mid and high notes. (Is not "surround sound" by any means)
Unless you have the proper equipment to delay the signal to the rear speakers then you are only destroying the image.
6x9 are not subs.
Toreador Red 05' FX4
4.0 4x4 A/Tran Ext.Cab Daily Driver. Nomads, BFG M/T's
Alpine 9857, JBL MS-8 DSP, US Acoustics 2x80w & 2x100w, Vifa 1" XT25 Dual Ring Radiator Tweeters 3k+ 24db slope, Dayton Audio 7" RS180-4 Mid-woofers 100Hz-3k 24db slope, 500w rms Kenwood Class-D Mono - JBL 10" GTO1014 Sealed. 100Hz- 12db slope