Hi Mike,
I've seen you've posted a question on this subject a couple of times now, so let me quickly explain what's happening with your older "fat" PS3 and the HD soundtracks on Blu-Rays. Basically, with the HD soundtracks (e.g. Dolby TrueHD and DTS HD Master Audio), these are encoded and they require decoding to
PCM in order for the amplifier to do anything with them. With most modern amplifiers and modern Blu-Ray players, there's basically two places this decoding can be done, either within the amp or within the Blu-Ray player. In this case, both your amp and your PS3 are capable of decoding the HD soundtracks to PCM. However, the older "fat" PS3 lacks the ability to send the HD soundtracks in their native form to the Onkyo amp (this process is known as bitstreaming). Because it can't do this, your best option is to choose
Linear PCM in the PS3's settings. This means these HD soundtracks are thus decoded onboard the PS3 to PCM and are then sent to the Onkyo amp.
In response, the Onkyo amp is detecting the fact that it's receiving a PCM signal from the PS3 and it's also detecting there is more than one channel. For this reason, it's showing MultiCh (or Multichannel PCM). This means the PS3 is therefore successfully decoding the soundtracks onboard and thus means you're getting the best possible audio format you can.
There is another way you can confirm this for sure, and that's to press the Select button on your PS3 controller (or bluetooth remote) when a film is playing. In the top right hand corner of the screen, it should display what soundtrack the PS3 is currently decoding. So if you're playing a film with a Dolby TrueHD soundtrack, pressing Select should display this in the top right hand corner of the TV.
I hope this helps, let me know if there's anything unclear!