I would like to add that I also have this problem. I made a big leap from MC12 to MC15; I'm always wary of changing stuff around because in my experience two things break for every one thing that gets fixed. I upgraded because I wanted the multiple server feature to provide optimal streaming for my mix of PlayStation 3, TiVo devices, Linksys wireless radios and computers.
Anyway my experience is consistent with what pjasper reports. TiVo works with MC12 but not MC15 (15.0.174). I've always had "always convert" because my music is a mix of WMA, AAC, and MP3. Playback starts but there is no progress, if I wait several seconds I get a quick burst then it goes to the next track, and the cycle repeats.
If I reset to "never convert" then I can get MP3 files to play only, on the TiVo.
Playback to my PS3 is fine, that is set to never convert.
Playback to my Linksys devices is also fine for the most part. And those always get converted, which strikes me as odd. I can play converted MP3, WMA, or AAC on those.
Also, my Media Center crashes out every time if I go to the Tivo Server page, stop server, change settings, and start server.
One other thing, the options for setting the encoder in Media Network seem a bit limited. On MC12 I used to use gogo with constant bit rate, it was really responsive and the Linksys radios never ever glitched with it. On MC15 I notice sometimes a song seems to end prematurely when I use the Linksys radios. I have it set to MP3 high bandwidth stream. Is that using LAME I take it? Is it CBR or VBR?
Media Center is running on headless windows server 2003.