I have been doing some testing with a new marantz 1603 and the standard method over port 23 won’t allow you to connect multiple connections at one time. The port 80 option configured as a Denon seems to work. What receivers have people tested that allow multiple connections at one time with full IP control?
Given that multiple connection handling via IP is available for both Denon and Yamaha as of Roomie 1.8, there is certainly much more support on the Denon side as that has been available for some months now and has had time to mature. It also has support for streaming feedback, album art, etc. Yamaha multiple connection support covers all the fundamental bases right now, but does not have streaming media feedback.
As far as other manufacturers go, most of them simply don’t offer such a control method. Marantz is the same as Denon of course.
Technically, it is a misstatement to say it is only supported with Denon and Yamaha. The Sony DN-1030 is also a multiple connection system. However, that receiver is so hobbled that it doesn’t even provide volume feedback via IP so let’s just not count it. Sony’s higher end receivers (that do of course provide volume feedback) are definitely single connection (oddly).
Onkyo (Integra) and Pioneer are not on the multiple connection map yet that we know of. Serial is your best bet for clean multiple connection support with those brands.