i-link being a completely jitter free connection. ... Perhaps this 'night and day' difference is most notable on Red Book playback.
I don't think jitter isn't the reason. Home Cinema Choice tested the audio jitter of the 868, the European version of the 59AVi, and they got something like 158 ps for jitter. That is *very* good. For whatever reason, Pioneer players always do well with jitter. I have seen tests of their CD players and even CD burners with really good jitter numbers (this goes back 8 years or so). I have also seen good numbers for the 3910 and the Onkyo 1000.
Plus, i.Link *can* be a jitterless connection, but isn't necessarily. You only get a jitterless connection by mating a Sony player to a Sony receiver with HATs (or something) engaged, or with a Pioneer player and receiver with PLQS (or something) engaged. There is a specific handshaking that must happen between both units to sync their clocks, and that doesn't necessarily happen with units from different manufacturers.
Also ... in each case where someone has compared the i.Link connection to 5.1 analog, neither of the following has been specified (that I remember):
a) Were the audio output levels matched to within 0.1 dB to create a fair test? Well known that a small difference in level can cause a person to think that a signal is "better" sounding, when it might not be.
b) Since i.Link brings the signal to the receiver digitally, that means the signal might be undergoing some kind of BM *and* TA in the receiver. Again, for the test to be fair, both the 5.1 analog output and the receiver's processing must be setup for "all large, sub on" with all speakers set to the same distance.
Obviously, i.Link's big benefit is being able to use the BM/TA processing in the receiver, and that creates a *different* set of audible benefits. But from purely an audio signal quality standpoint, at best, you are simply comparing the DACs in the player to the DACs in the receiver. Some player's DACs are better, some receiver's DACs are better, but that has nothing to do with i.Link itself.