Have you thought of trying Emm Labs? I've had ATC actives for 7 years (50's currently) and have tried and owned quite a few Dacs. I think I saw your question on the Naim forum re low level listening? I have an Emm Labs DV2i and one of the things I like the most about it is I seem to listen at lower volumes without loss of drive. I've owned or tried products from Naim. DCS, Auralic and others. Most sounded wonderful but not at all volume levels, I bought the (Emm Labs) Meitner MA3 originally as it matched my ATC 50's so well, this synergy led me to the DV2i.Thanks a lot. I’ll properly assess the SCA2. So far the TAD C600 brings out more musicality while remaining transparent and sounding good at low volume level. Of course the SCA2 is not broken in at all yet.
Emm Labs make a dedicated Pre which will have the same output stage. Definitely worth a thought.