The only Bb soprano Buffet R13 I've owned was purchased in January of 1991 in New York City, selected from nearly twenty R13s then in stock at Weiner's. My intent was to use it for classical playing, and it served that purpose for a decade, accompanying my journey through undergrad, grad school, and my first professional orchestra jobs.
Over the course of those years, I did my best to become a true "Buffet player", holding the instrument in a more tucked position and adjusting my embouchure and breathing accordingly. Because it was so different from the Selmer 10S jazz playing of my youth, switching back and forth was increasingly difficult. So the R13 became my all-purpose horn, including jazz, which is why this review is included here.
The chalumeau was a complete shift for me. Vintage Selmers, by and large, tend to cradle the sound in a different manner, so the player can hold the horn out more and blow in a more relaxed manner, letting the horn "catch" it. By contrast, my R13 tended to spread and be poorly focused with that approach. The well known "high tongue" position, pointed chin, and slightly pulled lips of American classical playing became very important to me for achieving a focused classical sound. This, by the way, seems to me the main reason Buffet players consider Selmers to be inferior and vice versa: the playing styles are very different and tend to interfere with each other (or at least they did--as of this writing, the most recent Selmer I've played was a 1999 Recital). At the height of my Buffet playing, I couldn't switch back to Selmers and get good results; the same was true the other way around.
The clarion was a good, solid R13 clarion, ideally used for blending in American orchestral sections, and useful for most situations, whether it was Prokofiev or Pops concerts. The timbral shift between the chalumeau and clarion was pronounced, though, compared to my Selmer 10S.
The altissimo was the weakest register for me. There wasn't one timbre, but many, almost shifting with each note. Flexibility was there, to a degree, but with a tone quality quite thin and shrill. The ultimate problem, possibly, was that I'd developed my altissimo approach on a Selmer, and kept trying to get that same quality.
That Buffets can and do make great jazz instruments is evidenced by some legendary recordings made by Artie Shaw in the 1950s, and Eddie Daniels in the late '80s and early '90s. Artie Shaw's final Gramercy 5 recordings were made on a Buffet, and these are, for me and many others, among the greatest jazz clarinet recordings ever made.
As for Eddie Daniels, albums like To Bird, with Love and Memos from Paradise helped listeners reapproach jazz clarinet after the instrument had endured quite a bit of obscurity. Eddie's Buffet altissimo, especially on the title suite from Memos, is breathtakingly beautiful and consistent.
Equipment and results ultimately depend on who is playing.
Bottom line: My natural playing style was at odds with the R13, which is why I sold my set and returned to Selmers. I would recommend recordings like the Shaw's 1954 Gramcery 5 sessions and Eddie Daniels' Memos from Paradise for examples of how they can sound at their best.