Amp/Preamps vs. High end AV Receivers

For your speaker surround system you don't really need AV separates cause they will cost you more. Just get a very good and very competent av receiver.
Get the Rotel RAP-1580 av receiver for your B&W speaker surround system. The Rotel will pair really well with the B&W. The RAP-1580 is Rotel’s only AV receiver model and it currently retails for $3800. It is a great sounding receiver and its stereo performance for music playbacks is pretty good especially coming from an AV receiver.

Or at cheaper price point I highly recommend the Cambridge Audio CRX200 av receiver currently retails for $2k. It’s pretty musical sounding av receiver. It will sound good on your B&W.

Guys, the OP has 5.1 setup that he uses for both surround sound movies and stereo music playbacks. He only has modest and mediocre B&W CM8 front speakers with a matching center speaker and the B&W CM5 standmount speakers for surrounds. He won’t need very expensive Gryphon Diablo integrated amp or some Vitus Audio or some other expensive electronics. They would be a waste unless if he’s willing to upgrade his speakers.

The Rotel RAP-1580 surround processor amplifier (AV receiver) will suffice it and will be a big step up performance wise or sonically from his Onkyo AV receiver.
I’m pretty sure this Rotel RAP-1580 will satisfy his needs and will pair well with his B&W speakers. Its performance for both stereo music and surround movie playbacks are very good especially coming from an AV receiver. The amplifier section is rock solid and this receiver is heavy weighs in around 54 lbs, unlike those Japanese big box mass produced AV receivers. However, the Rotel RAP-1580 is very barebone when it comes to features and bells & whistles.

Because a very good quality AV separates (av pre pro + multi-channel power amps) will cost him way more than this Rotel RAP-1580 av receiver plus multiple analog interconnects to connect between the two. Of course a good quality AV separates will be better than this Rotel RAP-1580 av receiver or any av receivers but he will need to spend quite a bit if he wants to get good quality AV pre pro and good quality multi-channel amps.

Or alternatively, he could get a good quality stereo integrated amp that has HT bypass and keep his Onkyo receiver for movies or surround duties if stereo is his first priority but it does not seem like he put priority on 2ch.

Even the Cambridge Audio CRX200 av receiver will sound very good on his speaker system. The Cambridge, along with Rotel NAD Arcam Audio Control, make quite musical sounding AV receivers and are quite a bit better than those mass produced Japanese AV receivers eg Denon, Onkyo/Integra, Marantz, Yamaha, Pioneer Elite & Sony.

The OP just stated that 2 channel is not his priority and he's using his system for mostly surrounds.

@atmasphere

I have no doubt that even modest speakers would benefit from high end electronics and cablings but some of the posters here earlier suggested the Gryphon Diablo integrated and some suggested Vitus Audio these are $20k+ worth of integrated amps and wouldn’t make any sense to pair them with the B&W CM8 unless if the OP is willing to upgrade his speakers. Besides the OP already stated that he wasn’t willing to pay a lot of money upgrading his electronics. He just needs something better than his Onkyo AV receiver.
And the Rotel RAP-1580 av receiver will be a big step up performance wise from what he has now.

Good qualittle AV separates will of course be better than AV receivers but will cost way more than AV receivers and if the OP doesn’t have budget constraint and yes indeed good quality AV separates will be perfect choice.
However, the Rotel RAP-1580 av receiver will most likely outperform entry level AV sepaeates. While the Rotel RAP-1580 isn’t exactly cheap at $3800 but its audio performance is very good and will sound really good on his B&W CM8 speaker system. I’ve heard this RAP-1580 paired with the new B&W 702 S2 speaker surround system as well as the B&W 804 D3 speaker surround system and I must say that this RAP-1580 beast drove the speakers with ease and sounded quite good especially coming from an AV receiver.

In regards to your comment that there’s always compromise in sound quality with integrated amps, I partially agree with you but really good design high end integrated amps are excellent sounding and there’s no compromise made.
But then of course when you compare them to their own stereo separate counterparts the separates will be better.

In my master bedroom setup I have the Dan D’Agostino Momentum stereo analog linestage integrated amp paired with the Wilson Audio Duette series 2 standmount speakers and they sounded excellent and outperforms many lower end stereo separates. And in my regular smaller sized bedroom I have the Naim Uniti Nova paired with the B&W 805 D3 standmount speakers and they sounded very good. Naim Uniti Nova is an all-in-one box solution streamer/DAC/integrated amp all in a single chassis. Very musical sounding unit for what it is.

However I do have very high end stereo separates (linestage preamp + monoblock amps) in my main reference two channel setup listening room.
I also have very high end AV separates in my dedicated home theater room.

@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.

@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. 

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.