Your question intrigues me and I'd like to propose one possible solution. Let me begin by saying that I'm a retired university professor who's been an audiophile for 40 years and was a silent partner in a high-end audio business for five years (from 1977-1982). (I no longer have any commercial connection to audio at all, so I'm not trying to sell you anything.) I've been to the high-end demonstrations at CES many times, as well as countless demonstrations in audio showrooms over a 40-year period. Like you I'm a classical music buff, with currently some 2,400 classical and opera recordings. When I used to be in the high-end audio business 20-25 years ago, with two partners, all three of us were keen on "the British sound," and we carried Spendor, Rogers, and Quad. We sold lots of Rogers LS3/5A's, Spendor BC-1's, and Quad electrostatics, and the then-new Quad ESL-63 when it came out in 1981. As you have no doubt learned, speaker design was a fundamentally mature technology a long time ago (as was amplifier design), and despite all the hype about terrific breakthroughs and colossal improvements in "the latest and greatest" gear from those eager to sell their new wares, a great speaker from 20, 30, or even 40 years ago is still a great speaker today (and the same can be said of great amps).
The Spendor BC-1 is one such classic speaker design, and I agree with the post that advises you of the difficulty of finding a new speaker you will find an improvement on the BC-1. I tried BC-1's in my home for a time, and I always liked them, but found I wanted something with a little more headroom and deeper bass, so I wound up with a pair of Spendor BC-3's, which I've owned and loved for 20 years and will never sell. This is the large, 4-way BBC studio monitor that Spendor made from 1980 to 1984. It was the BC-1's big brother and Spendor's top-of-the-line speaker at the time. It's a much larger, heavier (90 pounds each), costlier (they were $2,400 the pair 20 years ago) speaker than the BC-1, with an elaborate construction including a completely separate sub-enclosure for the 8" bextrene midrange driver (the same one that's in your BC-1), and a 12" bextrene woofer. This speaker solved the quibbles I had with the BC-1 (it will play louder and has much more extended, as well as better controlled, bass), but has all the musical virtues of the BC-1. It is one of the great, classic, full-range dynamic speaker systems. But it too is not flawless; it's problems are (1) that it's relatively inefficient, and (2) it evidently has a pretty horrendous impedance curve, because many amps sound unhappy trying to drive it (indeed, I've never heard another speaker that made differences in amps so dramatically pronounced). It has the further disadvantage of being very little known in the USA. (We knew the Spendor USA distributor pretty well in those days (he's now deceased), and he never made much effort to distribute the BC-3 here, perhaps because of the difficulties I've just mentioned.) Hence I hesitate to recommend the BC-3 to you. (And I assume that your problem with Quads, which you evidently admire (as I do), has to do with the fact that they are dipoles, because your listening room doesn't seem all that small, not with a 12' ceiling. I'm not quite clear on why the Quads couldn't be made to work in your listening room.)
I've got what may sound like a somewhat radical alternative to propose to you, but hear me out. You obviously like the sound of the BC-1's you've been happily living with all these years. My proposal is that, instead of spending a fortune on "new" speakers that you might wind up not liking as well, you acquire a second, used pair of BC-1's. You will need a second amp, and if you're pleased with the amp you've got driving your BC-1's, I'd suggest a second, used one. My suggestion is that you run "stacked" Spendor BC-1's, two per channel. The way to do this is to place the second pair of BC-1's on top of the first pair, but upside down (that is, with the tweeters next to each other). To drive the two amps you will need simple y-adapter interconnects from the preamp to the two amps; these are widely available.
I have considerable experience of "stacked" dynamic speaker systems, going back to the 1970s, when I started with a "stacked" system of four Large Advents (this was suggested by an article in TAS), then stacked Dynaco A-25s, then Dynaco A-35s. Today I have, in a secondary system, stacked Advent AS2's. In every case (always assuming adequate amplifiers), the stacked system has offered performance an order of magnitude superior to the single pair of speakers. The soundstage opens up dramatically, bass response is improved and extended (you'll have four 8" drivers moving air instead of two), and dynamic headroom is greatly increased, giving the system a sense of effortlessness and ease, especially noticeable in climaxes, that can't be approached by a single pair. The reason for this is obvious: all drivers, and both amps, are working half as hard to produce the same SPL, so everything is basically coasting, and there is no sense of strain when the music gets loud/complex. My experience is that most listeners who have never tried "stacking" are astonished at the difference it makes. It won't work with all speakers, but it will work with the BC-1's. It is essential, if you're going to try this, to invert the second pair and keep the tweeters next to each other; this will preserve the fine imaging/soundstaging of the BC-1's intact.
Finally, as the ultimate step in an improved system for you, I'd suggest a really good subwoofer. My own favorite is the Paradigm Reference Series Servo-15, a 90-pound 15" servo-controlled sub of superb quality, with an abundantly powerful amp built in. This sub goes all the say down to an honest 20Hz and has plenty of power for abundant undistorted low-bass output. It will extend the bottom end of the BC-1's significantly. I run one of these with Paradigm's X30 electronic crossover, which gives complete control over all parameters of the sub's performance (and you don't need to run the BC-1's through the crossover at all; preserve their purity by playing them flat out; that's what I do and there's not a problem). As top-quality subs and crossovers go, these two Paradigm products are bargains, and you should be able to find used ones at reasonable prices. But any one of a number of first-rate subs will work, and will extend the bottom-end performance of your system in a satisfying way to make it a truly full-range system.
With such a system, properly set up and adjusted, you'll have it all: the superb qualities of the BC-1's, a new ease and effortlessness of presentation with a dramatically opened-up soundstage, plenty of dynamic headroom, and true deep bass that you're not getting now (and I think you'll be surprised at how much deep bass there is in modern digital classical recordings that you haven't been hearing on the BC-1's).
