With a MacPro accessing my 60,000 songs at my Mom’s house (on Xfinity), I want to play on my Onkyo & AR9&90 system at my house (on Xfinity) gets me there?
Drobo thru Thunderbolt to MacPro … ethernet cable to Xfinity modem <-0->Xfinity modem (what box / device /software do I put in/on before my Integra DHC-80.3 9.2-Ch THX Networking A/V) AV preamp, 2 M-504 and AR-9 AR-90’s …my ears 
Your question is not clear. Are you saying that there is a library at your mom’s house running Roon and you want to access it at your house via Roon?
Yes. I want to access my Roon library through internet to another house. Can Roon work through my AV receiver? Do I need a DAC between ethernet and AV receiver?
It doesn’t work the way unless you have two Roon licenses and a way to VPN to the second location. Others have gotten this working here but without a serious amount of technical knowledge it might not be something easily accomplished.
Sounds like a challenge. Thanks for your help.
Do a search on this forum for VPN. That may help.