Sending native m4a (i.e., without transcoding)

Hi all-

Does anyone know how to configure Logitech Media Server to send native .m4a (mpeg aac audio) to Squeezebox Touch? I've spent hours messing w/ convert.conf, and I can get LMS to transcode the .m4a into .mp3 with faad and lame, and the Touch plays them happily. But the Touch is supposed to handle native .m4a. I don't understand what to stick in convert.conf to tell LMS to leave the .m4a alone and just send it off to the Touch.

FYI, the following works for transcoding:
mp4 mp3 * *
[faad] -q -w -f 1 $FILE$ | [lame] --silent -q 2 - -

For native support, I tried:
mp4 mp4 * *
-

All I see in server.log when I try to play the m4a natively is:
[12-11-30 17:31:44.5671] Slim::Player::Song::open (409) Error: Couldn't create command line for mp4 playback for [file:///home/media/x/01-mp4box.m4a]

Verion info:
Logitech Media Server Version: 7.7.2 - r33893 @ Wed Mar 14 06:31:06 MDT 2012
Player Model: Squeezebox Touch / Firmware: 7.7.2-r9663

Thanks in advance!
Rob