Proac 3.8 or Sonus Faber Cremona

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.

I recently sold my ProAc 3.8s as I am planning on upgrading to the new ProAc D80. I listened to to the Sonus Farber Cremona today at a local store and was simply blown away with their smooth life like sound. I can't say that they are better than my ProAc 3.8s were but they are extremely musical (and this with with a Plinus SS integrated amp). I do prefer tubes however and used an ARC VT-100MKII with my 3.8s. The Cremona has spiked my interest to the point that my local dealer is going to let my take a pair home for a week for audition in my system.

Go with the subwoofer approach. If you love your monitors and want more bottom end definetion and full range sound, doing a pair of subs like the Vandy 2wq's or Revel B15's is the way to go. The Revel B15 is probably the absolute best musical subwoofer on the market(and also the absolute most flexible too as it has unlimited room contour ability along with every input/output you would ever need) at ANY price, the Vandy is great for half the price.