Acapella vs. Avantgarde

I'm a longtime Avantgarde Duo owner (now upgraded to 108dB Duo Omegas) and can assure you that 50WPC is a great deal more than enough (my 18 watt Lamms are more than sufficient, power-wise, and I used to run 8WPC amps without power issues). The speakers are very directive, which eases setup in that wall/floor/ceiling reflections which plague cone speakers are not an issue, and if set up properly the driver integration is fine (the symptoms Billspecofc alludes to above are the result of improper adjustable crossover and/or level settings on the active subs; no such problems here). I hear no "shouty" horn coloration. For proper driver integration, you shoud sit at least 12 feet away (the speakers would then be 10 feet apart, toed in as Whart alludes to above). You'll be rewarded with (to my ears) the most lifelike power delivery and expressive ease you've ever known. I'm not personally familiar with the Acapellas, though it sounds as if their significantly lower efficiency may rob them of some of the specific virtues of horns.

I own a set of Acapella High Campaniles and can tell you that that I have never felt as if the sound was in any way disjointed. I have owned several speakers which did have problems melding different technologies but not Acapella.
OTOH they require power. I am not sure if 50 watts will do it. I would suggest an audition with your amps if possible.
Room placement is highly dependent on room size and acoustics but Acapella is not a difficult speaker to set up.

While I'm no expert on the Avantgardes (and have never listened to any Acapellas), I can tell you that when I auditioned a pr. of Duos I crossed them off the list because I could always tell when the sound went from the midrange horn to the woofers; it was not subtle. However, to answer another of your questions, they had no horn coloration, at least to my ears.

Can't speak to the Acapella but can give you insight into the Duo, which I bought a little more than a year ago, after more than 30 years of Quad electostat (57/Crosby 63) listening.

The horn 'honk' is virtually nonexistent if the upstream equipment is chosen carefully. For example, I was running a Steelhead straight in, without a linestage, and while the purity of the signal was undoubted, and the bass incredible, it sounded a little bright on certain passages, and sometimes shouty. After installing a Lamm linestage, the bass seemed to recede and the mids and highs got softer and more mellow. But, I don't think I am missing anything in the way of musical information by this change. And, the point here is that it is not a question of 'bad' equipment vs. 'good' equipment, but rather matching to the strengths of the various components.
Placement- I have a odd room, and am barely far enough away from the speakers at my seating area, but they are properly set up, according to some tricks from Jim Smith, the former US distributor and the image is uncanny. There is still a 'sweet spot' though, where everything comes together in correct perspective. (The Trick has to do with visually aligning the bolts on the stand from your listening position as a guide to toe-in- I can explain in greater detail if you like). The speakers, with the upstream equipment I am using, have a very dimensional sound stage, and image well, but that is not even the half of it- those are hi-fi virtues that are intended to compensate for the otherwise 'reproduced' quality of most systems. The thing I have been experiencing lately has to do with a non-mechanical continuousness in the flow of the music which simply does not sound reproduced in a mechanical sense. Much of this may have to do with the equipment upstream, including the Lamm amps. But, even as originally set up and before lot's of fiddling with associated equipment, the speakers have an 'alive' quality that transcends most hi-fi criteria - in this sense they bring you music. Not suggesting that the Acapellas cannot do the same, just limited to my experience with the Duos. The latter are very revealing, and unforgiving of any anomalies in the signal, or AC power, given their efficiency. You will have to work like the devil to lower your noise floor, but it will be well worth the trouble. Good luck.

Amfibius

I have heard the avantgardes a fair amount, a good friend has them. The acapella I have only heard a few times. I know the carry gear fairly well and it is lovely stuff.

From a practical viewpoint, the acapella are unlikely to be a huge improvement in terms of sensitivity. They are from the website 91 db into 8 ohms and recommended amps are 100 watts us. In contrast the duos (I believe the omega version) have a sensitivity of 107 db - 50 watts will deafen you if you wish.

As for coloration the only way for you to tell is to listen. If you are serious the dealer should be willing to get some in or at least listen to the unos. They will give you a very good idea of the speaker in terms of coloration.

Good luck

Gary