The differences in the 5508 models are only with regards to the rack ears coming with it or not, and the marketing agreements. The 5508 is available in the consumer Onkyo line, but doesn't come with the rack ears. Onkyo's corporate policy regarding direct dealers is rather old school requiring a commercial zoned B&M retail location. Those consumer Onkyo direct dealers with the 5508 must demo it and the new separate 9ch amp.
The 5508P is also available through Onkyo Pro direct dealers with traditional B&M locations. The 5508 is also available through distribution for custom installers, designers, etc. The seperate amp is only available through the consumer channel.
The 5508 is Audyssey Pro ready, although you don't see it on the Audyssey site yet. Audyssey Installers can acquire a digital key for the 5508 in the Audyssey Installer section.
I steered clear of the 5507 in our designs because it was not Audyssey Pro accessible. I don't know all its capabilities. We continued to use the 886 until the 5508 arrived because I always use Audyssey Pro for final calibration. The obvious Audyssey XT32 and DSX changes have been mentioned. Otherwise the significant refinements are the DLNA capability to be used with a media server and/or your PC, and ipod/iphone support from the front USB port. As many source products have this capability and more now, this is only a minor convenience. If you use a PS3, DLNA BD player, or use Apple TV to sync your itunes music library, photos, etc, these features in the Onkyo will not be of much use.
Anybody think of anything I've missed?
I took a look at the Denon 4311ci, too. Although admirable and very similar in their AVC functions, I would give the edge to the 5508. That said, I can see the 4311ci going in some designs. I will be changing out my 886 to the 5508.