Renmeister,
I totally agree with you that the vast majority of speaker systems do not deliver the kind of "drama" of live music the way the Avantgarde, and other horn systems, are capable of doing. I find that most systems sound either dead and constipated, or they attempt to inject some life into the sound with an artificial edginess. I also agree with you that Avantgarde speakers do have some problems with the bass response. The model I am most familiar with, the Duos, tended to overemphasize a certain portion of the bass range ("one-note" effect) and the bass did not integrate that well with the rest of the sound. I personally thought that the positive attributes far outway the bass problems so I like the speaker overall.
I don't think it will be easy to find a replacement for the Unos in your specified price range that will also deliver the same kind of lively sound. Also, one would have to consider whether a different amplifier will also be needed if you were using a low-powered amp with the Unos and the alternative were not as efficient.
You should audition Audionote speakers. These are fairly high in efficiency, and are quite lively sounding. But, in some respects, they too have a problem with the bass response. The bass can tend a bit toward being loose. Still, on balance, they are quite musical and are relatively free from the nasal coloration that is a problem with horns and other high efficiency designs.
If you can deal with lower efficiencies, try looking at speakers from the Triangle. These are quite lively sounding and quite musical. They are a bit bright, but, not in the harsh and unpleasant way many other speakers are also bright.
