I have the JDM Crx rear bass tubes in the stock location.
Would it be a good idea to put in some 6" midbass speakers in there ?
I currently have some 5.25" JBL coaxials in there for rear fill.

 

is the enclosure soft? maybe with some dampener inside and outside you could probably obtain good results.

This is to replace a sub right? bc if you still plan to add one i wouldn't use those and completely ditch the rear fill.

 

No not for sub ! These give the speaker more 'kick' midbass.
Im still going to need a subwoofer. I just wanted to ask if I should leave the coaxials in there
or use some 6" midbass woofer instead.

The tubes are hardend plastic with polyfill inside.

 

Well, it depends what you want. If you are building a competition SQ system, putting any speaker that close to your head behind you will be very hard to not localize.

If you're just wanting to add bump, then add a pair of decent 6.5" midbass to them.

 

^^second that!