Well the 816 has line level outputs which could connect to any line input on an Amp.
Many Surround Sound AV Receivers (Marantz, Denon, Onkyo etc ) include a set of Line Level Surround inputs ( ie L/C/R/Ls/Rs/Rs/Sub in the case of 5.1 ) to which you could feed 6 of your MR’s outputs. They are usually a set of 6 colored RCA/phono jacks on the back of the receiver … although not all have them. They were intended to connect the audio from DVD/Blue Ray players via analog. These days a single HDMI cable does the job! But a lot of Receivers still have the inputs.
The Receiver is then connected to passive speakers via its dedicated speaker terminals.Then in your DAW you could route your tracks when you want them to go per speaker channel/position.
The output of the MR ( +4 ) might be a bit hot for an AV receiver ( -10 ) but just back off on your output levels a bit.
I’ve done this on many installations/exhibits and found a consumer surround receiver the most economical way to ‘get there’. Sure you can find “Surround Monitor Controllers” and use individual amplifiers per surround speaker channel but unless you are setting up a serious surround studio its a pretty expensive way to go! ( I’ve done that too! )
Hope this helps