The ATCs are quite different to the Quads.  The Quads have more open sound and more precise crisper presentation.  The ATCs by comparison have more weight to the sound and less focused imaging.   I didn't really choose them for their similarity to the quads, but after auditioning several standmounts they were the ones I eventually felt most comfortable with and enjoyed most... though initially I has some doubts about them after the Quads.

The speaker I tried which was probably most like the Quads tonally was the Sonus Faber Concertino, incredibly good value for only £600 new... very precise imaging and direct presentation of the sound.  Eventually I preferred the slightly more comforting tone and warmth of the ATCs.  Since changing the amp to the Tact any suspicion of bass flabbiness with the ATCs has been dispelled.