Greetings Don222,
That's a tall order you're trying to fill. I can't think of a tougher act to follow than trying to match the original Quads in the midrange. Subsequent Quads fall short, as does just about everything else out there.
When I go around shopping for speakers at CES or whatever, the standard for comparison I use is the original Quad ESL and the big Sound Labs electrostats (I own both and sell the latter). That being the case, there are very few dynamic speakers that have come close. I've heard three speakers priced under five grand that are worth mentioning, and I chose to sell two of them.
When I heard the Cliffhanger Bulldogs, their voicing and general presentation actually reminded me of the orginal Quads. That was a first. Now, they don't have quite the sweetness of the Quads, but they do have a similar lively sense of presence.
The Heil Kithara has a lot of the timbral and textural richness of the Quads. It also has a much wider sweet spot, which might be quite welcome in a large room. I think the Heil driver is one of the finest unconventional drivers made. If the Quads give you a front third of the hall perspective, the Heils give more of a middle or back third of the hall perspective
The Gradient Revolutions were actually designed with the Quads as the reference speakers. They are essentially boxless speakers in disguise - the bass is dipolar and the midrange uses a pressure-relief box that allows most of the backwave radiation to escape through an acoustic resistor. They among the most unboxy sounding dynamic speakers I've heard, in this respect equalled only by the Audio Artistry Beethoven (in my experience). The Revolution's voicing is very smooth and relaxing, and the perspective is a bit farther back than what you get with the Quads.
Frankly I didn't hear anything between five and ten grand that significantly improves on the midrange presentation of these three. Any one of these will give you goosebumps if the recording warrants it.
If you have a large room, I would think the Kithara would do the best job of filling it to fairly high sound pressure levels. Or you could go with the Active version of the Revolutions, which would allow you to add additional bass modules to increase the system's macrodynamic capability.
Have you considered stacking two or three pairs of original Quads? That might take up a bit more real estate than you'd planned, but the midrange would sure be omigod lovely.
Best wishes to you on your quest!
