I also have a 640Cv2 CDP. It is a detail champ with a fairly flat frequency response for the money, but benefits from good synergy with other gear. I listened to my player with some excellent Moon electronics, Dynaudio Contour speakers and Shunyata wires (some wires costing significantly more than the player) and it was shocking how good the Cambridge performed in that company.
My system with more modest electronics and speakers can sound a bit forward at times, but I prefer the resolution of the Cambridge to other players in its price range (including Arcam). I bought a Shunyata Diamondback power cable after the demo referenced above and that helped every aspect of the 640C's performance (couldn't justify spending more than the player cost on a PC - even though it sounded better). I also ditched several more expensive IC cables for the previous generation of the current Radio Shack Auvio's (RS Fusion's actually) because I feel they work better with the Cambridge and my modified AMC amp. I really like these wires and think they are a screaming deal at around $20.
I'm not sure the budget V-DAC is the answer given the quality of your other gear - I would try before you buy. Or try some different wires (including an aftermarket power cable if you don't already have one), and save major money for a stand alone DAC that is really a lot better than the two already used in the Cambridge. I have often thought the 640Cv2 would make a good transport for something like the Berkley or Bryston.
I have heard the Epos M16i's with Naim electronics and it was a really good combination to my ear, nicely balanced and fairly neutral, showcasing the Naim's PRAT and musicality. I prefered the combination to Naim's own speakers and several Totem tower's, only prefering the much more expensive Totem Forest in that particular setup. I would think your Quad amp would be a good match for the Epos as well. Do you usually play the Epos with the grills on by the way? That might help tame the treble a bit - and how much have you fooled around with speaker placement and listening position? In any case, if none of those suggestions work, the Cambridge CDP may ultimately make you happier matched with a silk dome tweeter rather than the metal one in the Epos.
The Quad 22L2 has more bass and may suffice in your room without a sub. I really, really like the 21L2, and I think you gain some coherence with one fewer driver and crossover. But it may not do in your room, especially without a sub. The Quad towers also sounded great to me with Naim electronics - almost as good as Devore 8's which I was comparing at the time and cost a good bit more. And the fit and finish of the Quad's is ridiculously good for the prices asked - Chinese manufacturing at its very best...
Good luck