What vintage speaker might you use today

Trelja, good response! It sounds like you found your sweet spot for enjoyment. That's what it's all about. I'm a variety person; there's no one speaker technology which completely satisfies me, regardless of it being reportedly SOTA.

Not picking an argument with you, but a response to your statement, "My priorities lie in the sonic realm." Allow me to regale you with a story of subwoofers. Very fine adjustments of the Legacy Audio XTREME HD subs (as do all subs) influence the sonics of vocals, even female vocals, chamber music, you name it. I was dialing in the Vapor Audio Joule White 3 speakers today and tweaking the subs using female vocals and the music of Musica Nuda. Very light adjustments on the subs refines the timbre and spatial aspects of the voice and upright bass. If you have the means and space, I encourage you to consider adding sub(s) because it will elevate the experience with the Quads. If you select them well and carefully dial them in it will not harm the sense of speed and openness, but it will make them sound like they are innately more capable speakers. 

I will never forget the day many years ago when I visited a high end shop, I think it was Music for Pleasure in St. Louis. They demonstrated the benefits of a huge REL sub with a pair of capable floor standers using female vocals. Sub out - sub in - sub out... and my Immediate conversion to the principle that subs make a world of difference in the soundstage and timbre of vocals, or any other music. It almost seems magical, the power of a sub at discrete level to enrich music across the spectrum and elevate the performance of an already favored speaker. I invariably use them with the Kingsound King III ESL, even though it reaches 28Hz. The addition of the subs makes them far better sounding holistically and improves their spatial characteristics. I always review speakers independently of my subs, but almost invariably when the "testing" is over, the subs go in again and the experience is elevated. 
All this is all independent of listening level. I also do not typically listen beyond 95-97dB.   :) 

Well said trelja! You (and I, and ct0517) are not alone in finding enduring merit in the original Quad when used within its limitations. Yes, those limitations are severe, making it unsuitable for many applications and listeners, Douglas Schroeder apparently included. But for "small" music (Baroque, Chamber, Vocal, acoustic Jazz Trios/Quartets/Quintets, etc., Bluegrass, Folk, etc.) at modest (though sufficient) volume, the Quad is still not just unbeaten, but, say some, unequalled! A fair number of loudspeaker designers keep a pair of quads around with which to help voice their own designs, and recording engineers still use them to check the naturalness of their work, vocals in particular (ask John Atkinson).

One sonic aspect as heard via some of the vintage speaker designs, their reproduction of bass compared to modern designs, gets an interesting comment in below quote.

[...] all this talk about bass started me thinking about some of the speakers I have reviewed and listened to at audio shows in the last couple of years. A few came to mind: the Burwell & Sons Homage speakers, the big JBL Everest, the incredible sounding RCA LC-1A LS-11 and those wonderful sounding Tannoy Golds mounted in a Jensen Imperial Cabinets turned upside down so that the empty horn part of the cabinet acted as a stand to raise the Tannoy Gold drivers to the right height. Those were in the Pass Labs room at the 2014 California Audio Show.

All of these speakers have several things in common. First, they are all based on or actually are speakers from the mid to late 50s. Second, none of them attempt to play down into the 20s, in fact some don’t make it below 45Hz. Third, they all have very large drivers, most of them have 15-inch bass drivers and the Quad ESL bass panels have around 500 square inches for each speaker. Lastly, while they don’t all sound the same in the bass they all sound wonderfully musical.

There seems to me to be something fundamentally different in the way these speakers play bass compared to modern speakers with their super dead cabinets and incredible fast, tight and really deep bass. While these speakers sound very impressive their bass just doesn’t flow within the performance like these older-design speakers. The bass on these newer speakers is definitely deeper, faster and has more slam, but they just don’t have the life in the bass that the more vintage designs do. All of the speakers above have incredible air and harmonics in the bass. You feel the bass. Yes, you feel the bass with the modern speaker as well, but differently. The bass from modern speakers with extremely dead cabinets has a very pistonic sound. To me, real music seldom sounds this way, occasionally rock music does, but it also often sounds purposefully distorted.

http://www.dagogo.com/beatnik-pet-peeve-3-way-modern-speakers-play-bass

I can attest to the merits of vintage designs incorporating large bass units and their supposed "musical" imprinting. My recently acquired all-horn loaded speakers use 15" bass drivers in folded horns, and are a pretty radical departure from my previous bass reflex-loaded speakers in providing what can actually be described as timbre and tunefulness in the bass with a seamless blend to the mid horn above them. They are specified to reach 56Hz only, a likely reason why many an interested audiophile would probably discount them prior to any audition, but the physicality, effortless power, speed and gently pressurized "wavefront" here presented is so tunefully imbedded in the remaining frequency spectrum above that it makes any preoccupation with bass extension per se seem utterly misplaced. I didn't know that prior to listening to them and how much the quality (and type) of bass could truly matter, so much indeed as to come to eschew most of what I've heard of the typical bass in modern designs with smaller (usually reflex loaded) drivers. I can still enjoy such more modern bass designs for what they are, but it's with the proviso that I probably wouldn't want to own them.

As I was composing my post directly above, Doug posted his. Also well reasoned, I second his recommendation to augment the Quad with a pair of subs. But not just any sub---the GR Research/Rythmik OB/Dipole Sub. It is in some ways similar to the one Gradient designed and offered for the Quad ESL63 in the 80's and 90's, but quite a bit better. It is particularly well suited for dipole loudspeakers, sounding very different from a "normal" sub, no matter the quality. You can read all about it on the GR Research website and in their Audiocircle Forum. Very special!

Ok, this is fun. I don’t even love some of these, but have good memories. Does that make sense?

Klipsch corner horns. These were the first high end speakers I ever heard. It was in 69 at HiFi Stereo House in my home town of Newington, CT. They were run with phase linear and crown gear if I recall correctly. I would love to tinker with the crossovers using new components and internal cabling. I’d also play with the veneer as I work with wood and would have a ton of fun finishing these honking horns (I typically don’t even like horns, lol).

Proac Response 2’s. These were a great speaker in the day. They could be a bit hot on top, but man did they disappear and they did midrange correctly. Very listenable and easy to drive with top tube gear.

Quad.....

ET’s.

JSE infinite slopes. I think it was the 5’s that were the large ones. 5 drivers if I recall. Those really sounded great with modded B&K gear from a store in Bristol CT who specialized in modding the B&K line. Many nights with wine and music in the store, lol.

Of course the last ones would be the Vandersteen 2's with NAD separates from 89.  All being fed from a Sota vacuum TT with a nice Van Den Hul cartridge.  We finally had real cable in 89 too ;)....