I have both of these Panasonic units CY-AC300 and CQ-DVR909 (not in the same car). I use the 909 more so let me tell you about this unit in a minute.
First off the center channel is very important for dialog in DD/DTS 5.1 configuration. Most of the movie audio content is on this channel! The 909 built-in center channel sucks. Not only because it's not loud enough (adjustable up to 10+), but it's 'thin' in sound. Your whole front soundstage is shot using a good left and right front speaker. The 909 does have a center channel pre-amp output that can be turned on/off (on/none) and adjusted for Large (full range speaker) or small (limited range speaker). I'd take advantage of that if your serious about DD/DTS in the car.
The most useful feature for DD/DTS in a car is "Dynamic Range Control" DRC and "Down Mix". DRC makes louder passages softer and softer passages louder. The Down Mix converts the 5.1 mix into 2 channel stereo. I know this defeats the purpose for DD/DTS, but it works really good for noisy road conditions and just watching a movie. The Panasonic 909 has both these features, I'm not sure about the CY-AC300. I do remember the AC-300 has a proprietary optical input, so it must be used with Panasonic mobile DVD player and the 909 has no digital (optical or coax) for DD/DTS output.
Another benefit of the 909 is it plays DVD-Audio discs. They sound really good. Panasonic also has a few newer DD/DTS headunits you should probably look at and compare against the 909.
Hope this helps,
Steve
