Hey Graham
Make sure firmware is updated contact support@bryston.com if you need help with that.
You must use Pre Out from a Home Theater Receiver (HTR) or Preamp for front Left and Right into one of the Analog inputs either XLR or RCA (don't convert XLR to RCA stick with what HTR outputs). (I am assuming you are already using line outs of BR-20 to stereo power amp and your front left and right speaker wires are already connected to that power amp).
Once connected make sure volume on HTR is turned down low. Then select that analog input you connected HTR to. I use RCA 2 which Bryston would refer to as Single 2 in Dashboard.
You push down code button on remote hold down until light blinks white quickly (5 seconds) then quickly enter D2, D2, D10 on BR-20 remote, the input light you selected should turn red (was green) or pink (was blue). The volume control light will also be at 2 o'clock position. (if it failed you might find one of buttons on remote is stuck down unstick and retry).
You only need to do this one time, once you have converted the selected analog input it will always be HTR Bypass until you reset unit. If you connect a fixed output device like a CD-Player to that input you will likely blow speakers, if you don't give someone in house a heart attack SO BE CAREFUL.
You can undo HTR Bypass by selecting the analog input you converted and repeating the Code 220 until input light reverts and volume control lowers from 2 o'clock position.
FYI sometimes need to redo after a firmware update. Bryston firmware update instructions will be clear on that make sure to follow to letter for the version of firmware you are updating (instructions are in same directory as firmware).
Then select an active input on HTR, increase volume slowly and you will have sound.
I highly recommend you output some test tones from the HTR (or use a setup disk) and use a Sound Pressure Meter to adjust volume on different channels so everything is balanced and level. I do not find these setup Mics connected to HTR do a good job of this, its a place to start but not very accurate. I never use the EQ settings of setup mic just the level adjustments. I have had a Radio Shack unit for decades. (buy an expensive Lithium battery if you get one they never leak like regular batteries).
I am using a Marantz 7012 which like many high end units has Pre Out and 7.1 input so confirm connections are Pre-Out Left and Right to selected analog input.
James if you rather people contact you for this info let me now I'll edit post.
I have a 9.1 setup so two pairs of Dolby Atmos speakers in addition to regular 5.1. Some movies and the Br-20 in stereo, really freak the poor dog out, he can't figure out where the sound is coming from. If a dog barking he looses it stops searching and goes upstairs. (Pink Floyd - Animals)
Playing Diablo 4 on TV too, lots of dogs barking in rear surrounds.
Jerry
FYI my first HTR a Denon I bought used, he said the bass was terrible, I updated firmware and proceeded to use my SPL meter to setup channels, bass was -20db it wound up at -3db so much for setup mics. My sub volume or level was about 11 oclock so just under half. Even if his sub was at full level he should have been at -10 to -8.