Loudspeakers have we really made that much progress since the 1930s?

Yes and no.  It would be very difficult to find whole speaker systems, and even modern speakers using vintage drivers, that would make compact monitors that sound as good as modern monitor speakers.  The same probably holds true for thin column-type floorstanding speakers.

But, for systems where one is not constrained by practical considerations, such as size and ugliness, I would go with a modern system built with vintage or replica vintage drivers over anything I've heard from modern speakers.  If size and cost is not an issue, something with Jensen M-18 fieldcoil woofers and Western Electric 555 drivers and 15A horn and 597 tweeter would be nice to have (and a room big enough for such a system would be really nice).  For a "compact" system, something with the Jensen/ERPI M-13 fieldcoil and a 597 tweeter would also be a nice choice.  I also happen to like my system which is built around the Western Electric 713b midrange compression driver (made sometime around 1939).

At the Capital Audiofest, held this past weekend, Deja Vu Audio was showing a recently-built speaker featuring vintage drivers.  The speakers had 15" Jensen woofers and midrange compression drivers from Yoshimura Labs (a 1960-70's Japanese company that made theater/public address systems that emulated Western Electric drivers) in an open baffle configuration (it looked like a box with a closed back, but the back allowed sound to pass through).  It is hard to beat this type of system for delivering harmonic "denseness" (rich, saturated sound) and a relaxed (not edgy) and natural presentation while also delivering terrific dynamics, speed and clarity.

The BIG plus with many vintage systems is that they can be run with a wide array of low-powered amps.  To me, the best sound is delivered by lower powered amps, regardless of whether one goes with tubes or solid state.

As usual LarryI has a balanced and measured post which means I agree with him.  There is no question that modern speaker design software and measurement tools have contributed to an overall improvement in speaker sound quality.  For those of us who were kicking tires in hifi stores in the 1960s and each decade since, I can say that there are fewer poor sounding speakers today than in years past.  This is especially true with lower price speakers and smaller speakers. 

But if we ignore price and size considerations, the very best vintage drivers and horns have some special musical qualities that are not equaled by today's speakers at any price.  I have only limited experience with the big Western Electric horns (and no experience at all with the Shearer horns mentioned by the OP) but each time I have heard them left no doubt they are capable of excellent sound quality.  On a much more accessible level, I find that my own speakers using a vintage 15" Jensen paper cone woofer and Altec 32A horn with 802-8G compression driver (dating probably from the 1970s) are very satisfying musically. 

Sal,
Yes, Larryi comments are consistently insightful and well-thought-out.  Larryi in general I agree with you preference for the lower power amplifiers in regard to superior sound quality. This point is driven home often when I hear the mega power amplifiers driving difficult load/inefficient speakers. 
Charles, 

Regarding Thiel loudspeakers, I certainly hope so. Happy Listening!

The Theil-Small parameters is something that has changed! It makes predicting the performance of a driver in a box possible. Prior to these specs, sorting out the right size for a port opening in a box was experience and guesswork. This has lead to wider range loudspeakers with less coloration.

Another thing that has improved is horn design. Its now possible to optimize in a way that was not 50 or 80 years ago. There are also improved materials for compression drivers, so its possible to build smoother sounding more detailed horns. My speakers at home benefit from this- the first breakup in the midrange horn is at about 35KHz. So it has the speed and detail of an ESL.

 It led me to information I didn't know about the Hartsfield, probably the third super speaker system I encountered as a kid after the Electro Voice Patrician and Bozak Concert Grand in the early 50's.

Classic Audio Loudspeakers has been making a Harsfield reproduction since the mid-1980s. The proprietor, John Wolff, has improved the detail and bandwidth by using materials and drivers that did not exist in the 1950s. They play bass much better than the originals! They are smoother and more detailed as well as being more efficient (I'm sure the field coil drivers he uses are part of that).

Its a simple fact that loudspeaker technology is significantly advanced over what was around even 30 years ago. Materials and modeling have gotten better and I expect that will continue.