Bi-amp ? in your setup, it is not getting you much benefit
Step 1) Do you have ECO Mode or set the impedance setting to 6 ohm in your receiver? or something else that painfully gimps your system? These 2 settings will badly current limit the amp sections and make it sound bad. If you did, turn off ECO mode and change impedance back to 8 ohm.
Do you have adaptive DRC enabled? Kill it for 2 channel. It is just going to abuse your 2 channel listening experience.
Step 2) Set DSP to 2 channel stereo and use bass management. Take some measurements and
manually adjust PEQ to taste. Save those 2 channel peq settings and other 2 channel optimization settings under one of your "pattern" settings for switching back and forth between 2 channel and 5 channel easily. I really have no idea why some dudes suggest "pure direct" as soon as they hear 2 channel stereo. Your receiver came with extra goodies like bass management and PEQ. Use them.
If step 1 and step 2 didn't satisfy you, it could be that the preamp/dac on the RX-A1080 is not cutting it for you.
In that case, get this integrated amplifier if you have more cash and connect it to your Cambridge streamer for 2 channel listening.
https://www.adorama.com/yaas2200sl.html
You can connect the power amp main-in (bypassing its preamp) of this amp to your receiver's 2 front channel preouts when you're in 5 channel for multichannel music or hometheater.
The above mentioned setup could be referred to as a 2 channel/multi-channel hybrid system.
Mid priced option,
https://www.adorama.com/yaas1200sl.html
Lower price option but still plenty good...( Anything lower than this won't be a step up from the RX-A1080),
Get this integrated amplifier and connect it to your Cambridge streamer. No preamp bypass option on this .
https://www.adorama.com/yaas801sl.html
All 3 of these integrated amps should manhandle you and your speakers....show you a good time
If none of these integrated amps paired with the Cambridge streamer satisfy you, toss your Monitor Audio speakers when the trash truck stops by and get the Elac Unifi 2.0 series speakers (lower cash option), JBL HDI series (higher cash option) or something better....