Just got my 550 delivered today. I hooked it up via HMDI to my Sony 5300es.
The 550 seems to be internally decoding TrueHD and DTS MA and sending LPCM to the 5300es. I popped in Nightmare Before Christmas (DD TrueHD 7.1) and it will send 7.1 LPCM to the receiver.
However, with Rambo (DTS MA 7.1) it only sent LPCM 5.1
Golden Compass (DTS MA 7.1) also only LPCM 5.1 (interestingly enough punching the display button on the 550 showed DTS 3/2.1)
Things started to get weird when I set the 550 to direct.
With Nightmare Before Christmas, there either wasn't any audio or the audio cut in/out. The receiver shows it thinks it's getting DD TrueHD 1.0.
Same with I Am Legend (DD TrueHD 5.1) and Resident Evil: Extinction (DD True HD 5.1).
10,000 B.C. (DD TrueHD 5.1) the receiver showed DD TrueHD 3/2.1, however there was no audio heard.
Rambo did show up with DTS MA 3/4.1 on the receiver and sounded good.
Sushine (DTS MA 5.1) also showed up as DTS MA 3/2.1 on the receiver.
Golden Compass showed up with DTS MA 3/4.1 on the receiver (and interestingly enough the display button on the 550 now show DTS Master Audio 3/4.1)
So some things seem to work....some don't. I'm not sure where the problem lies...the 550, the receiver or the titles. Looks like at least in my case if I want 7.1 from a DD TrueHD disc, I leave the audio setting in the 550 at "Mix". If I wannt 7.1 from a DTS MA disc, I have to set it to "Direct".