I used to have ATC 35 which are very similar. My room is almost twice as large and still I had some problems below 100Hz with a powerfull amplifier like ATC, Bryston or Chord. The manufacturer told me it sounds best in a 16-40 sq. m. room, which in my estimate is a bit too liberal. I might say ideally they need smth like 25-35 sq.m.
I am a bit surprused that you are using it with Primaluna. I am not sure they are able to control ATC woofers, their magnets are so heavy. I think now that in a smallish room these ATCs would need quite a powerfull SS amp, let's say 100+ watts which is able to firmly keep the bass at bay. With hindsight I think ASR Emitter would do the job nicely in my room, not sure about yours.
No, it's too tricky to have these speakers in such a small room.
I think Reference 3a or a small Sonus Faber is a good idea.
Room treatment might help in theory- but you will need to reduce your small room considerably to get a worthwhile effect. We are talking 40 cm. deep bass traps or some expensive things like Bekeshi etc. A rug on the wall may give you the opposite effect - it will damp highs and some upper mids if it thick enough, and subjectively there may be even be MORE bass. It will just sound overdamped and dull. A rug is absolutely transparent for low freqs.
Having said all that rooms are funny things - sometimes they work against all odds. Wall material, ceiling hight and material are impotant.