Quad ESL 2805 or Avantgarde uno Horn?

Another consideration is: How much power do you intend to drive them with? The Avantgardes are extremely efficient 100db/1w/1m, and can be driven by low powered amps(ie: SETs) to loud levels. As mentioned: They will act as a magnifying lens toward any shortcomings in the rest of the system. Personally I consider that a plus, since any system shortcoming will distract, color or otherwise hinder my musical experience anyway(even if subliminally). Some prefer to remain in the dark about that(ignorance CAN be bliss). Are Quads excellent speakers? Absolutely!! The Unos have a much wider dynamic range, can play cleanly at VERY low or VERY high SPLs(unlike the Quads), but also as mentioned: horns are never very good at depth of sound stage, or pinpoint imaging. They aren't as sensitive to room placement either. If you enjoy the bottom octave of bass(40 to 20hz), both will need a subwoofer. Avantgarde has a massive, matching horn available, IF you have the room. DO take the time to listen to them. Hopefully- the system(including cables) and room acoustics the owner has will do them justice. Then it's a matter of whether their presentation is your cup of tea. If you enjoy actual concert SPLs, you'll like the UNOs. Chances are good that the room the Quads were auditioned in was treated well acoustically. That might give them an advantage, if the home you will listen to the UNOs in is otherwise. Prepare to do some acoustic treatment in your home, whichever you go with. Much of the above has already been stated. I just wanted to encourage you to give them a listen. I've been a horn-hater for over twenty years. New horn technology has modified my stance, given the wide dynamic range of live music/horn systems. I'm still listening to actively bi-amped planars though(I'm addicted to the sound staging).