Need Help! In Dash Stereo Upgrade

[LEGA-Z]

I am currently having difficulties with my upgraded sound system. I have an 09 370z with the Nav system. I chose to keep the Nav instead of installing an aftermarket stereo, because I preferred the look of the Nav, but now I feel like my sound quality is suffering because of this.. My subs are not hitting nearly as hard as they should (on single bass hits), but on rolling bass (like most Dubstep songs) they perform just like they should. They seem to "build up" to the bass hit, instead of just going with the hit... It's kinda like bbbbbbOOM, bbbbbbOOM.... Instead of just BOOM BOOM... Get it??

My question is if there is a way to install another stereo while still keeping the stock Nav... I would give up most functionality, like bluetooth, voice navigation... Etc. I just want to be able to search for directions... The new stereo would be installed right under it.... I want to know if this setup is possible.

Or any other suggestions to fix the problem...