<font color='#000000'>I can answer part of your question (or I should probably say that Yamaha can answer it!). Here is a quote from p.11 of the RX-V2400 Manual:
"Presence speakers (PR and PL)
Presence speakers supplement the sound from the front speakers with extra ambient effects produced by CINEMA DSP. These effects include sounds that filmmakers intend to locate a little farther back behind the screen in order to create more theater-like ambience. Place these speakers at the front of the room about 0.5-1m (1-3 feet) outside the front speakers, facing slightly inwards, and about 1.8m (6 feet) above the floor."
I have not yet set up speakers to use the presence option. Initially I had a 5.1 setup (with a sub) and then went to 7.1. When I added the two speakers for the 7.1 config, I think I detected a slight loss of horsepower in the system so I'm not that anxious to add the presence...
I have not actually listened to a system with presence speakers so I cannot comment on the effects. On the 2400, it works only in DSP modes.
-GeorgeM</font>