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.