Is Rotel RMB-1075 a good match for B&W N804

If possible try the Parasound P5. I had one and for the money it just may do the trick.

Bypassed the Yamaha and plugged the Oppo directly in the Rotel, see a big difference. Overall sound quality is very good, see a gain the midrange but not enough in the bass. Thank you. Which pre-amp (in the $2000 range)would you suggest that can go well with the B&W and Rotel gear that I can use for both music and HT ? How about the Emotiva XMC-1? Or should I look at Parasound P7.

Home theatre receivers, in general, tend to perform very poorly as preamps. The Yamaha would definitely be a major problem for sure.

The Oppo has its own volume control. Try plugging it directly into the amp so that you bypass the Yamaha. You should hear a very big difference.

Oppo BDP-103 and Yamaha RX-V650 as the pre.

What's the rest of your system? The problem may lie elsewhere.