I'm thinking about adding a pair of midbass speakers and a small-ish sub to the back of my E53 X5. I've got the non-DSP/non-NAV system and while I don't find it completely horrible, I'd like just a little more bass and a little fuller sound.

My thought was a pair of 6.5" midbass speakers in the cargo area. The left one would go into a small box in the space where the CD changer lives. On the right side, I'd like to build an enclosure that I can use for a 6.5" midbass and an 8" sub.

Is putting those in the same box a horrible idea?
I'm not looking for ear-bleeding window-shattering bass... just a little more punch.