I had the regular Treo's and LOVED them. I now own the Quatro CT (painted in Audi Havana Black). There are differences in the drivers for sure. That ceramic tweeter in the 5a is a great tweeter, but the CT tweeter does take things to the next level.
Personally, the Quatro really is the sweet spot in the lineup. I chose a new pair of Quatro's, from Johnny Rutan, than to get a used pair of 5a's. I do have a large room adn it's very open so bass is a very important thing to me. That's why I went with "Quatro over the Treo's.
The 5a won't overload your room. They can be played in smaller rooms and still sound great, but to me the Quatro is easier to place in the room and it has plenty of bass. It too has the 11 band EQ, so it's a trade off between MORE bass (5a's) or much more refined sound (Quatro) that still have plenty of bass. I personally side with refined when I can afford to so take my comments in context. Feel free to PM me as I love talking audio, lol...Pete