@terrancej
If you only add a power amp and use your Onkyo receiver as a preamp processor/DAC you won’t be able to achieve optimal improvement in performance cause you’ll still be using your Onkyo receiver as preamp processor & DAC. Preamp processor & DAC plays very important role in determining the overall sound quality of your setup system. One must upgrade the front end components (DAC & preamp) first before upgrading the power amp, not the other way around. What’s lost earlier in the signal path cannot be restored downstream.
I know you’re on a budget and I can recommend a slightly used Marantz AV8802 av preamp processor (discontinued) which can be had today at slightly less than $2k used. its original retail price was $4k. This pre pro supports 4k video, hdcp 2.2, Dolby Atmos DTS-X & Auro 3D. This will be a step up sonically than using your Onkyo receiver’s preamp/processor section.
The AV8802 sounded very good for the money and I would say it is the best sounding Japanese mass produced pre pro. Noticeably better than Onkyo/Integra, Yamaha or Denon processors.
And pair this Marantz AV8802 processor with the Rotel RMB-1585 five-channel power amp and you will be set and will drastically elevate the overall performance of your system to a whole new level of sonic performance.
The Rotel RMB-1585 five-channel amp is a class AB design amplifier and it retails for $3k new but often times you can find a used one in used markets for around $2k ish. This is a spectacular sounding multi-channel amp especially for the money and will sound way better than the Emotiva, Outlaw, or B&K amps or any other amps mentioned earlier in the above posts.
However, do not get the Rotel class D amplifier such as the RMB-1575. it isn’t the same as its class AB design such as the RMB-1585.
Or alternatively for your B&W CM8 speaker system you can get the Rotel RMB-1555 five-channel amp, which is a step down from the RMB-1585. The RMB-1555 is also a class AB design. The RMB-1555 retails for around $2k ish brand new or it can be had for around $1300 used.
Rotel amps will sound really good on your B&W speakers. Rotel always pair really well with B&W. They are meant to be paired together. B&W speakers (especially the CM series & 600 series) were voiced using Rotel amplifications. They have great sonic synergy together.