I've been using my new Yamaha VX-R2400 for a while and on a for what it's worth basis wanted to pass along my impressions of the various surround modes. I have a 7.1 channel setup. Not Axiom, but very good speakers.
After listening to a lot of music on CD's, I find that I like the Neo 6 a tad bit better than the PLIIx, however, I'm finding that Straight Stereo, which takes all the DSP out of the signal path is far superior to either, and I'm finding that I'm gravitating to doing almost all my music listening in that mode. While the surround modes do give some ambient sense, they seem to reduce/fuzz the performance soundstage, and some fine detail seems obscured. Straight Stereo brings it back, and the increase in front soundstage and audible resolution more than compensates with the slight ambience loss.
I'm also finding that when watching TV, I seem to like the Neo 6 cinema setting. In this case, the surround does seem to work giving me more of a 'being there' feel and the overall sound seems more natural. The PLIIx is also good in this respect, but tonally a bit different- I happen to like the Neo a tad bit better, but both are better than stereo on TV.