Anthem 225 vs Parasound A21 vs Emotiva Xpa 7 Gen 2

ARC CD5 and Rowland Capri

Dali MS-4 and Aerial 10T

Khartago Plus or Extreme can be had for price of A23 (benefits of mfg direct), no idea which he will prefer, just some options to check out in his price range

I haven't heard the Cambridge 851 but I have an 840W which I sometimes use directly with either an Upgrade Company or a Modwright modded Oppo 83SE.  The Modwright had the outboard tube power supply while the Upgrade Co is solid state.  I have a pair of BG Corp Radia 520DX loudspeakers, which is a hybrid ribbon speaker.  Either player sounds nicely detailed and smooth with the Cambridge amp.  Obviously the Modwright is warmer and fuller sounding bordering on overripe sometimes but always musical.  So, if the 851 isn't too different from the 840W, I'd say you should be quite pleased with it (or the 840W for that matter).  I did have a pair of Emotiva XPA-1 gen 1  briefly and they are not in the same league as the 840W at all, being significantly less resolving, much less dimensional (2-D mid-fi), and loose in the bass.   A couple of things to consider  with this/these Cambridge amp(s): they require a 20A plug at the amp end and run very warm to almost hot.  

What speakers are you driving with the (integrated)amp? I know you said ribon tweeters. I did mention integrated as I know you talked about amps driving directly from your oppo but have you thought about the flexibility of an integrated so your not tied to that oppo player for everything in the future or do you plan to add a pre amp later? I don't know much about the oppo players do they handle the volume control in the digital domain if so do they truncate the bits to vary the volume. If so you'd be better with an integrated amp and using the oppo directly out. Maybe someone else can chime in that knows more about the oppo players.