Amp/Preamps vs. High end AV Receivers

when I replaced the front channels on my Onkyo AVR with a same power Parasound amp, I was quite surprised at the huge difference in sound quality, both for music and movies...

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.

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.