I owned Quad USA Monitor Pros for 6 yrs. These are similar to the Quad 2805s. Quads are superb speakers, with a cult-like following in the sense that people who have or had them LOVE them. Quad owners are on the good side; they are not equipment'philes or bass-heads, etc; they are music lovers. Likely Quad owners go to or have been to lots of concerts.
For whatever the reason, I've never heard Quads perform up to their considerable potential at dealers or at shows. And thus I am wondering if the 2805's you heard and liked were heard in someones home. Its been mentioned in bulletin boards that Quads are difficult to set up effectively. I never thought so. Pay attention to the details common to setting up any speaker. After that, catch the 1st reflection points with room treatment, room treat some of front wall behind the speakers, and don't put any objects in between the speakers.
As has been mentioned, Quads are somewhat shy in upper frequencies and in the bass. Their recessed upper end is not noticeable or is hardly ever noticeable likely as the lightness in the bass serves to not highlight the recessed high end. It was only after listening to my next speaker that the recessed high end in the Quads became obvious. Another thing about the Quads, should they ever require repair, QS&D on the east coast and Nick Gowen (True Sound) on the west coast provide their clients with some of the best support in the industry.
Something that may sway your selection between the Quad and Avantgarde is the type of music you listen to. The Quad's limitations, their lack of bass, lack of small and large scale dynamics, their inability to play reasonable loud limit the types of music played satisfactorily. These characteristics may not affect you if you listen mostly to rock, folk, jazz, vocal, world music, and chamber. If you enjoy orchestral music or organ music, these limitations at some point will be bothersome.
I have a different take than Electroid who guessed that if one likes one of these speakers they'd not like the other. I own Avantgarde Duos which share many similarities to the Unos. The Duos, when set up correctly and used with synergistic equipment, share the magic that music lovers love their Quads for but adds a lot more to the table. Should a Quad owner decide they want that next step that gets them closer to the real thing, Avantgardes can provide that next step.
The Duos do not limit the types of music played as all types of music play well. AGs are full extension, no upper or lower frequency limitations. Large scale and small scale dynamics are literally breathtaking. When you walk by an open window where music is playing, it is easy to determine if the music is live or reproduced. It is the inability to reproduce dynamics, shared by many systems, that is the give away. When AGs are playing, your mind has to work a lot harder to decide if the music is live or not.
The AG's ability to capture subtle dynamic changes is as important if not more important than their way with large scale dynamics. Listening on the Quads to someone like Sinatra or Ella with their amazing instruments, the essence of these singers comes through loud and clear. Superb sounding voices are in between your speakers. The Duos add to this the subtle low volume vocal characteristics that grace their vocals. Instead of just the essence of their singing, you hear all of their singing--this is a huge difference. Instead of having superb sounding voices in between your speakers, you are now sitting in the club where they are singing. With classical, in part due to the emotion involvement gained in hearing the subtle details that other systems ignore, I'm anxious to get home for a listening session after leaving a symphony concert.
With the Quads, guests would listen and be awed with the sound. And why not? The sound was more music-like with none of the showy but ultimately grating characteristics heard from some systems. Non-audio types were stunned that a stereo could reproduce music so well. With the Duos, reactions were dramatically different as guests felt they were participates at a concert. At times theyd clap after a song or an LP side finished.
Regarding some of the comments in this thread. Someone implied that the Avantgardes cannot be placed close to a sidewall. My recollection of their dispersion characteristics is that they do not project much sound in that direction. Thus, I think the AGs can be placed close to side walls.
Per the mention that AGs do not image, soundstage or focus well. That is incorrect--don't blame the messenger! If AG's youve heard are lacking in those areas, there is something amiss in the electronics, front end, cables, room placement, crossover adjustment, etc. My AGs have created a large rectangular soundstage with images properly placed all over the stage including way behind each speaker.
George, good luck on your quest. You are considering 2 fabulous speakers, there are no wrong decisions.