I will try yo answer some of the questions posed...
Prices are listed on the importer's website at http://www.aaudioimports.com/
We are an Acapella dealer, and we get asked this question, to compare Avantgarde and Acapella, all the time, probably because of the apparent visual simularities (and the fact that they both start with the letter 'A' and they are often displayed with 'sportscar red' horns, doesn't help any).
Compared to the Avantgarde, Acapella's horns have a more substantial construction and design which eliminates horn colorations and resonances.
The higher end of the Acapella line also incorporates a plasma tweeter which is generally acknowledged to be one of the better, if not the best, tweeter currently available.
The integration of the drivers and driver technologies is also done somewhat better by Acapella, as well.
On the other hand, Avantgarde usually has tighter and more forcefull bass response and is usually quite a bit more efficient and can be driven by very small, sweet tube amps (although the top speakers in the Acapella line get up to 98-100 dB, which ain't too bad either).
Just our opinion, of course, but the top-of-the-line Acapella speakers are *so* much better than any other speaker we have heard, and we've heard darn near all of them, that it is somewhat disturbing and really makes one completely re-evaluate the meaning of musical expression and experience.
Well, on that metaphysical 'note', ;-)
-Mike (Acapella dealer)