I have no problem with 5 Sonos zone players syncing, with one ZP80 into my 568.2, another into active speakers, and 3 more ZP100s into passie speakers.

What DSP preset do you use in the 561? Do you have, by any chance, a frame delay set in the 561? (just had to ask).

Good luck.

Main system: Cable, Apple TV, HD621, G68D, AC11, 218, DSP7200SE (L+R), DSP3200 (L+R Rear)
2nd system: Cable, Apple TV, MC200, HD621, 568.2, M3100 (L+R)