I have owned both of these exact speakers as part of the parade of small speakers that have passed through here during the past several years. The Duets I just sold, the Ref 3A's were several years ago.
They don't sound very much alike but they are both wonderful and superbly musical speakers -- musical meaning they communicate in a way that grabs you and pulls you into the music. I think Art Dudley's review of the Ref 3A in Stereophile was spot-on. And so was JA's measurements write up, in which he noted a mid-range rise. In my experience with it, this rise was a problem for me, but I can understand why many would be fine with it . The Ref 3A's have a big, open, dynamic sound that is, overall, forward in character -- there is a lot of imaging going on in front of the speakers. They have excellent and very adequate bass and a smooth, open treble. They play all kinds of music well, as I recall. I wouldn't mind having them back for another listen. They do very well with tubes.
The Duets, in my experience, are more restrained and have a smaller sound. They sound very immediate and have a "close up" perspective. JMR talk about them being both quick and warm, which I agree with. They have a woody warmth similar to Harbeths but are faster and more exciting. I described them as restrained compared to the Ref 3A, but they are less restrained than Harbeths. A bit less detailed and refined also, I would say. I used them on Skylan stands, which I am sure is quite common. The wonderful Bob Neill of Amherst Audio, who sold them to me, said that JMR's Magic Stands would be a significant upgrade, extending the bass and eliminating a bit of thickness that I heard. Both of those improvements would have been very welcome, but I never tried the Magic Stands. I used the Duets mainly with an Ayre integrated. I'm sure there are much better matches out there (Blue Circle is supposed to be the bomb), but I was not able to do much experimenting with amplification while I had the speakers. In my setup, the Duets were best with classical and other acoustic music, less good with rock.
By the way, my room is 17 x 23 and really needs a bigger speaker than the Duet. In a smaller room, they could be perfect. Ditto for the Ref 3As.