Amp/Preamps vs. High end AV Receivers

I strongly suggest you check out a Classe Sigma SSP Pre/Pro.  It was designed on B&W speakers when Classe was part of the B&W family.  Classe was shut down but I understand it is back up and running.   Balanced for L/R channels, single ended for rest to economize.  It partners great with a Classe Amp 5 (2 balanced or SE, 3 SE channels) for very good sound.  You should be able to get both for close to $5K used. You will likely want to upgrade your primary amp in the future but then you will have a really great 7.1 setup if you want.  There are a few of each of these currently available on Audiogon.  If you want to really round out the system, add an Aurender N100H!  Enjoy!  

@terrancej 

I wouldn't recommend the Integra DHC 60.7 processor. It wouldn't make a difference from the preamp processor section of your Onkyo receiver. 

The Classe Sigma SSP preamp processor and the Classe Sigma Amp5 five-channel amp that was recommended by psnyder149 above are great but they will cost around $6k used altogether in used markets. However, the Classe Sigma Amp5 is a class D amplifier, which is very good for HT but not so for music IMO. But the Classe Sigma SSP preamp processor is excellent and is optimized for stereo. It originally retails for $6k for the Sigma SSP Mk2 and $5k for the first early version of the Sigma SSP. 
The Classe Sigma Amp5 five-channel amp retails for $5k new. 
If you can afford this Classe Sigma combo it would be great and will be big step up sonically from your Onkyo AV receiver. 

In my opinion one cant compare an AV receiver to separate components.The detail with HQ separate components is in a class of its own.However there are some HQ av's.Problem is most have hdmi decoders with the audio logic control.
DD, DTS...I see many every month with blown hdmi, cant be repaired and no spares available a day after warranty expires...RIP A/V!Rather purchase an arxam/cambridge/rotel...and use HQ add on's.Separate components are easy to work on and most can be repaired for life.

As others have mentioned, upgrading the amp without upgrading the preamp will only get you so far, and upgrading to separates will be pretty expensive.  Given your investment in speakers, if it were me I'd upgrade to a higher level AVR so you upgrade both the amp and preamp in one shot.  Anthem makes relatively high quality AVRs, so I'd look for something like this if it has the features you're looking for.    

https://www.audiogon.com/listings/lis974j9-anthem-710-receiver-receivers

If you want to go further than that, I'd do a Marantz prepro with maybe Rotel or Classe multichannel amps, but it will cost a lot more and you'll need to add good interconnects.  FWIW, and best of luck. 

Yup....do not underestimate the important role of a preamp or pre pro. This is one element that is often times overlooked by many.

If you can swing around $5500 or so I would recommend the Classe Sigma SSP preamp processor, which is optimized for stereo, with the matching Classe Sigma Amp5 five channel amp. However, like I said before the Sigma Amp5 is a class D design amplifier which is good for surrounds (HT) but isn’t my preferred choice for music. But the Sigma SSP is an excellent sounding pre pro especially for stereo music playbacks. This combo can be had in used markets for around $5500 to $6k altogether both used plus analog interconnect cables to connect between the two. Do not skimp on cables.
Classe also makes class AB amps called the Delta series which are excellent for both music and HT but they are way more expensive than the Classe Sigma Amp5 or any Classe Sigma series amps (class D).

Or you can pair the Classe Sigma SSP pre pro with the Rotel RMB-1585 five channel amp, which is class AB design amplifier. Actually I have heard this combo before at someone’s house and they sounded very good together especially on B&W speakers. The Rotel RMB-1585 amp will pair really well with your B&W CM8 and they will sound great together.
The Rotel RMB-1585 retails for $3k new and sometimes you can find a used one for around $2k ish. It is a very good amplifier very refined sounding and very musical especially for the money. The Classe Sigma Amp5 (class D) is more suitable for surround movies or HT than the Rotel but the Rotel RMB-1585 is more inviting and engaging for music.