Hi Loganfan. I think the Sonos is the weak link in your chain. Oppos are known for their very good DAC’s and are audiophile grade components where as the Sonos Connect is a mass market consumer grade product and are not known for High fidelity sound. "Wyred 4 Sound" sells a modified Sonos Connect that sounds much better than the stock version but there are newer and better "audiophile" products that you should look at. I’m sure you will get many good suggestions here soon.
I think you would have to spend more than $1000. on an out board DAC to much improve on the OPPO. Also don’t be afraid to shop used for a DAC. Digital is evolving very rapidly and this is very good for used prices of older gear.
I can recommend the Wyred 4 Sound DAC2 Very solid full featured DAC. It’s a few years old now but sold new for $1500. and sells for $500. - $600. used. Several are available currently. Check the reviews. They are up gradable to newer versions as well including current top of the line. Excellent value! http://https//wyred4sound.com/products/digital-converters/dac-2v2-series
Good luck.
I think you would have to spend more than $1000. on an out board DAC to much improve on the OPPO. Also don’t be afraid to shop used for a DAC. Digital is evolving very rapidly and this is very good for used prices of older gear.
I can recommend the Wyred 4 Sound DAC2 Very solid full featured DAC. It’s a few years old now but sold new for $1500. and sells for $500. - $600. used. Several are available currently. Check the reviews. They are up gradable to newer versions as well including current top of the line. Excellent value! http://https//wyred4sound.com/products/digital-converters/dac-2v2-series
Good luck.