maybe your question is not very clear....do you want to watch a movie with 5.1 audio in the living room AND music via HEOS in other rooms?.... or do you want to add HEOS speakers to your 5.1 and possibly make a 7.1 system?... but maybe this page will help you...(https://support-de.denon.com/app/answers/detail/a_id/5791/related/1/session/L2F2LRVj8NqmcnHFztp53C1pSdpVjsf9ANBCtpYHpjq37JK9cLiYk1XDENujcj59gtcbX17gkTNBZHowb05FdThEazdzWHB2N1J4Q0V6U1d5aiU3RUs2OGJoRVpPQ2VZaUNhOUolN0Vj8NqmcnHFztp51K6UGbHi8bXraLRVj8NqmcnHFztp53C1pSdpVjsf9ANBCtpYHK9K90hPaW5fQ1VMelZ3JTIxJTIx)

Unfortunately, your question is a little unclear and it is not clear what equipment you already have (monitor, speakers, etc.). Please state everything and explain exactly what you want to achieve so that they can help you.

Something with the speakers. See manual p. 185: https://assets.denon.com/DocumentMaster/CHG/AVRS650H_EU_DE.pdf

You need a Y-Cinch cable (mono to duo), e.g. this one: https://www.digitec.ch/de/s1/product/sonero-subwoofer-stereo-mono-y-kabel-150m-entry-cinch-audio-kabel-14804100. The Denon reciever has 2 subwoofer outputs, each with a RCA socket.