The Denon AVR 1908 doesn't have preouts for the main zones, only for zone 2.
So the only way you can connect an amp is to use the zone 2 output. And if you use zone 2 then only analog sources will work there.
You can't biamp the RM6750 speakers, they only have one set of speaker terminals on the back. your front speakers are only getting partial sound and same with the surrounds. Its all messed up!
You definitely have your system set up incorrectly. Turn off the biamping feature.
If you only have a 5.1 setup you use surround speaker terminals, not the surround back. The surround back terminals are for 7.1. And definitely turn off the bi amp feature.
If it has Bluetooth its gonna have all the networking streaming stuff too.
Doubt you'll find anything in white or silver. At least not in north america.
Look at the Yamaha RX-S600
Or the Marantz slimline receivers.
Either center channel? The AVR 254 only has one set of speaker level center channel outputs.
perhaps you need to set the center channel to YES or ON in the setup menu?
Do you have the owners manual? If not get it here.