Easy way to find out, since your @TV and rDAC are very portable, is to take them along to a Sonos dealer and see if the extra cost would be worthwhile.
Personally, I would predict not. You've done everything you can to get the best quality out of the @TV by adding the rDAC, and it's only the musical ability of your receiver which could hold it back now.
Might I suggest upgrading your toslink cable from the ATV. I switched out a cheap monster cable for a wireworld supernova6 and it truly made the sound more transparent and open. It's a stumper and defies logic, but to my ears the cable change made an improvement well worth the investment.
Oh,and my Apple TV feeding my DAC is outstanding. My CDP is now for sale. I cannot imagine how I could improve the sound.
I agree with JD, you need to look at your receiver as the weak link at this point.