What you've encountered here is one limitation to using a PS3 as a BluRay player. It cannot output HD audio (DolbyTrueHD or DTSHD) in a raw (bitstream) format. All it can do is process the signal and output it as PCM. I'm pretty sure that it can do 7.1 channel PCM, but I know from personal experience that it can do 5.1 PCM.
So you're still going to get the soundtrack in full, real, and exquisite surround sound, even though your AVR just says 'PCM' instead of 'Dolby Digital' or 'DTS' or whatever.
With that 72TX, it doesn't matter anyway because the 72TX doesn't know how to decode a DD-HD or DTS-HD bitstream. So even if the PS3 could output the bitstream, you wouldn't be able to use it with your receiver. You'd have to tell it to output it in PCM.
