Amp/Preamps vs. High end AV Receivers

Integrated Amplifiers are another story ,seperate almost allways are better 
do to separation of RF, and noise , and bigger power supplies and capacitance .
that being said .my brother has the latest Gryphon Diablo ,and it is almost 85 lbs 
and Everything Premium ,and I am into system modding it is a work of art 
and will compete with ease Any seperates in the $20k plus range .
an integrated can have advantages if properly implemented such as no extra cables ,losses in signal there direct wiring and coupled  can have its own synergy 
with amp preamp in one package .$17k is not cheap and their $4k digital module 
is Excellent . If he ever sells it .i will take it Off his hands . I have the Very good Danish Gato 400-S Digital integrated amp that I feel a true bargain at $6500
i  have spent a homes worth on Audio overthe years and owned a audio store 
in The U.K . I feel well implemented class A,AB, Digital ,and Vacuum Tube 
all have examples that are excellent . To date the Gryphon  integrated 
is the most musical integrated I have heard. Vitus  from Denmark another Great equally as good
hand built integrated, their top seperates are better stil  if youhave $40k and up 
I donot .  A McIntosh receiver is very respectable , Accuphase much better still.
we are speaking of High quality gear. Denon, onykyo is ok for a starter system 
Audiophile quality they are not.

I was in your spot debating my options. I went with a MAC6700 integrated receiver. I am totally satisfied. You can find them in the $5K range

B&W CM8 specs:
88dB spl (2.83V, 1m) which is like approximately 86 dB/1 W/1 Meter (quite low).
Impedance: 8Ω (minimum 3Ω) (the minimum does not look good).

Not sure how far you sit from the speakers if one assumes 9ft, you might need for the amp to be at-least putting out 96 Watts of clean watts to reach 85 dB + 15 dB (swings) all the way to 3 Ohm when needed
http://myhometheater.homestead.com/splcalculator.html

So an amp that probably does 200 Watts per channel so that THD is low at 100 Watts, typical of many amps, there are exceptions however.

I don't think there are many receivers that can go up-to 200 Watts per channel and also support impedance to 2 Ohm (at-least on spec).

Better go to with separates for 2 channel an HT bypass will come in handy. This will depend on how you source the music and the music from the videos.

check out emotiva

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.