Nab2,
Great thread idea - I was also shopping in this same ballpark recently. Here are the options that stood out to me in my search, for one reason or another, which fit into your proposed price range (I am not saying I loved all of these speakers, but they all offered some qualities of note):
Avalon Time
YG Anat III Studio / Signature
Magico Q3 / Q5
Wilson Alexia
Tidal Contriva Diacera SE / G2
Kharma Midi Exquisite
Marten Coltrane
MBL 101e
Apogee Full Range
Sound Lab U-1
With that said, I have found that speakers are oftentimes the products most subject to an individual's tastes, and thus auditioning is a must to find the speakers that work right for you - so I strongly encourage you to hit the show when and as you can. For example, there are some products referenced in posts above for which I cared very little, but that does not mean they are invalid choices, depending on what is of musical importance to you. Also, don't discount the need to have the appropriate equipment (particularly amplifiers) for driving your speaker of choice - some of the above speakers require real horsepower, and you may end up spending as much in amps as you will on the speakers themselves.
"Bang for the buck" is, as lawyers are wont to say, a bit of a "vague and ambiguous" term. Do you mean absolute cost v. performance? Do you mean cost savings at used prices relative to retail? Or some combination of the two? Of the above, one could say the Magicos offer the worst bang for the buck, but that is only because they hold their retail price value very well. Thus, you can expect to pay >50% of their retail price (e.g., $22-25k for a pair of Q3s, which retail for ~$45k). Conversely, the YG's may offer exceptional bang for the buck, as Anat II's that retailed for >$100k can be had for $25-28k. But in absolute terms, which is a better value for the money at that price? Without knowing your particular tastes, it is hard to say (I would lean toward the YG's, if you forced me to pick). As another example, the Apogee full ranges are fantastic speakers, fully comparable with others on the above list, and can be had for ~$10k (if you can find them) - thus,
in and of themselves, they would seem to present the best value. However, you really need to triamp them with amps that have exceptional current capability to get the best out of them - a popular combo is with 3 pairs of Mark Levinson ML-2s. After you factor in the the cost of all the amplification needed, they are not so much the bargain as they would seem.
If I had to pick one speaker that would be the best "bang for the buck", I would actually go lower in price and point to the Genesis 200s / 300s / 350 SEs - at their used prices, they are incredible bargains, easy to drive, easy to place, full 20Hz - 20kHz frequency performance, tremendous soundstage depth and dynamics (although a little constrained in soundstage width). However, note the original Genesis is no longer around, and I do not know if the new Genesis offers support for these older models.
Of the particular models I listed above, I will only say the speakers I ended up buying represented the best bang for the buck to me, but that is only because of how good deal was (which also is a luck of the draw type of thing); several other speakers that I listed, in my view, actually offered higher performance, but at (significantly) greater cost, even used.
I don't think you need to make the "jump" to $50k if you are buying used - in the right system, any one of the above can provide a level of sound quality that represents an end of the road type investment (i.e., no more upgrade bug). The only exception would be if you have a truly massive listening room (which you don't seem to have, based on the above) and/or are a pipe organ fanatic - in that case, as an example, a Wilson Alexandria may be more to your tastes than the Alexia.
Best of luck, and let us know how your search progresses.
