Great commentary re this most wonderful chamber work and great to see the clarinet get some love! A few additional comments; some, related to the already mentioned facts.
It is interesting to note that early in the evolution of instruments commonly used today such as the clarinet, there was often little that was “standard” in their design as there is today. As has been noted, during Mozart’s time and for some time after, there existed many “varieties” of clarinets, pitched in different keys and with different ranges. Many became obsolete over the years. Players and/or composers would often commission instruments with unique design characteristics in order to satisfy compositional needs. Btw, to fine tune (😉) twoleftears’ excellent comments on this:
The clarinet variants that still exist today and their most commonly used range “designations” are piccolo clarinet (Ab), sopranino clarinet (Eb, D), soprano clarinet (C,Bb,A), alto clarinet (Eb), basset horn (F), contra alto (Eb) and contra bass (Bb). On a related note, it is interesting that both Mozart’s clarinet quintet and concerto were written in the key of A. The basset clarinet which Stadler owned, and as they are today, was pitched in the key of A. Mozart’s choice of the key of A for these works was surely because doing so would make the execution of technical passages much easier since, as has been noted, early clarinet variants had few keys; making difficult passages much more difficult or impossible if written in a different key. Btw, the modern basset clarinet looks nothing like the instrument that Stadler played. Today’s basset clarinet looks just like a “regular” clarinet, but with an extended lower joint and the necessary additional keys. Btw, the clarinet got its name from the Italian “clarino” (trumpet). Early clarinets had a trumpet-like tone quality.
Historically informed performances are often very interesting and some are excellent. In this case the extended range of the basset clarinet most certainly adds a degree of compositional authenticity to a performance. However, performances/recordings made with a regular A clarinet (no extended range) should not be dismissed as necessarily inferior. In fact, in my favorite recordings of this great work the basset clarinet is not used. Purists should consider how much performance practices have changed over the years. As inconceivable as it may seem today, but in typical fashion for the time, at the premiere performance of this work the piece is reputed to have been performed one movement at a time, with other works or movements of other works performed between them!
Probably my favorite, Karl Leister with the Vienna Quartet is wonderful, as is the mentioned Harold Wright. For a great example of a rendition with what could be called a more modern approach to clarinet playing, Anthony McGill with the Pacifica Quartet is great. The recording by the lesser known early 20th century Austrian clarinetist Leopold Wlach is wonderful for its very relaxed feeling and recommended….if you can find it.