Acoustic Zen, Reference 3A and ATC

Acoustic Zen would be my pick. I also feel the 3a's are a little forward sounding. I am not a fan of any of the ATC speakers as they use the cabinet to act as part of the sound. I personally do not want my box to add its own colorations to the music.

I don't think any of these speakers are particularly colored in the midrange and all are great in their own way.  But there will be differences in how they present the mids and the overall sonic picture.  If you're into soundstaging and, in particular, 3D imaging (think more of a "you are there" presentation) I'd lean toward Ref 3a.  If midrange dynamics and impact are more important to you (think "they are here" presentation) I've not heard better than ATC.  A major consideration is what equipment you'll be driving the speakers with, which will also be a main contributor to having listener fatigue or not.  ATCs generally need more power to open up and sound their best where Ref 3as present a relatively easier load and require less power.  I haven't heard the AZs so can't comment, but I'd guess they're somewhere between the two.  Others I'd consider are Joseph Audio, Vandersteen, and ProAc.  All great speakers.  Hope this helps and best of luck. 

Soix speaks the truth...amp/power is a must with atc...im running the modwright kwi 200...still have to give em the juice...also room consideration...

+1 Soix