Fascinating,
I own the XA777ES (10 years old now, I guess) and the XA5400ES and much prefer the newer player. Well, it's hardly new - three years old.
In my system, CD playback on the 5400 is significantly better than the 777ES; and SACD is somewhat better on the 5400. My 777ES was moved into the bedroom system when the 5400 arrived.
The build quality on the XA777ES is far superior - the machine is a tank. But for sound quality, the 5400 is my choice.
This player really takes a long time to break in and show its best.
Regards,