Proac 3.8 or Sonus Faber Cremona

I used the 3.8 with ARC VT100 MkIII and loved the result. I ultimately traded them because they don't really open up at apartment listening levels (i.e. they need to be "cranked" before sounding balanced). Their bi-wireing scheme was also not to my liking in that one set of binding posts feed ONLY the tweeter -the other feeds midrange and woofer. I'm used to a tweet-mid on one post and woofer on the other. If these traits aren't a concern, they are great speakers!

The Concentra and the Proac 2.5's or D15's sound great together. But it might help if you share why you want to change speakers. What are you looking for that you don't get with the EAII (which BTW,I also love with the Concentra)?

The principal problem its a lack of bass output efficiency.
Another problem it's when i'm listening to the system during night and i can't go to high in volume then the sound looses only bass output.

I would recommend a musical sub like REL or Vandersteen if everything is great except for the bass ouptput at lower volumes. The vandersteen 2wq sure helps me with my PROAC 1.5's at lower listen levels and even higher ones.

I think Fletcher-Munson has a role in what you're experiencing. Other than a separate subwoofer, as Aniwolfe suggested, allowing you to control the bass volume separately, or a tone control, I don't see a solution.

I also hazard to guess that the one thing that we rarely do, but should, when auditioning speakers is turn the volume down. Many speakers have a tendency to sound balanced at a certain volume "sweet spot" but colored at lower or higher volumes. Try it when auditioning your replacements.

Good luck.