Sorry about the belated reply, I got sick, ended up in hospital and totally forgot about this post.
Most of the large AAC files are tagged as being created by Audiobook Builder in the metadata, ~64kbit/s 32khz ... my wife managed to collect a couple hundred gig of drm'd books with her audible.com account, I found out about that when her ipod died and bought us a clip-zip each (cause I wanted something that ran Rockbox). Since then I've been tracking down drm stripped copies of her collection, slowly working through it until it'll all play on the clip-zip, duration wise the files are running around 12 hours.
For the time being I convert to mp3 and they play fine, but obviously it'd be cool if no conversion was necessary.