I use my n900 in the car for phone, sygic maps and music.

Audio quality via bluetooth carkit is poor, would like to route music output to FMTX and leave phone on BT.

Any suggestions?

It's possible but the FMTX deactivates whenever there is a phone call. I personally use the FMTX for podcasts/music and leave the bluetooth on and connected to my headset for incoming calls. I just have to turn down the volume on the FM radio in my car whenever a call comes in (otherwise it's just static since there's no strong radio station on the frequency I use). After the call ends, the media player resumes and I turn the volume back up on the car stereo.

There is an odd issue where for a second (while the N900 is reactivating the FMTX after a call) that the audio goes into the bluetooth headset but then it returns to normal.