Flairdj, I don't there is a straightforward answer to your question.
For Cyrus:
Right off the bat, for pure stereo listening your Onkyo is a weak link (as are most A/V amps). Now since you have a pretty high-end CD player, I can't say whether getting a decent outboard DAC (you'd have to spend well over 300 quid IMO) is gonna give you much improvement, considering it's still going into the Onkyo.
Alternatively connecting your Cyrus to your Onk's digital input is a no no, I doubt the Onk's DAC is as good as the built-in Cyrus for pure music.
For Sonos:
IMO, getting a Sonos Zone Player, connecting it to a decent DAC (say the inexpensive Beresford or 200 quid Cambridge Audio DACMagic) and into your Onkyo might actually give you decent results.
Sum-up:
I would say get a dealer home and audition the Sonos, this is the best way forward IMO. Plus you if you won't use it you can flog your Cyrus for a tidy sum.