I received the 8400 about a week ago. My review only relates to audio, as I have yet to connect to a monitor. This also contrains my review in the sense that I have not been able to tweak most of the settings, which require a monitor. For example, this has limited my DVD-A listening to 96 kHz.
First off, forget anything you heard about the 8300. If this is Marantz's mea culpa, then they hit the mark (though you may not feel this way if you bought an 8300!). Sound is distinct, rich and full throughout the sound spectrum. I prefer not to use bass management, and cannot speak to this. But the sound in each format is simply marvelous IMO.
Is it worth it? At this point, I feel the quality is there. If you wait, no doubt prices will fall. On the other hand, why be limited in what you can listen to? It's the software, after all, that we pay to listen to.
Lastly, to confirm a rumour, the DVI port is blocked at this point "for future upgrade." Legal or standard hold-ups, I assume, though Marantz REALLY should disclose this on their website. While certainly the kHz contraints on non-RCA/ digital output are really very silly, at the end-of the day they don't hold back that marvelous sound from those very same ports, so personally I wouldn't get too caught up in a knot over it.
Best - Kahana