And to that pair of quartets one could add the Balanescus and the Smiths and the Binghams and countless others who've commissioned and otherwise championed new quartet writing. Is there any other genre of chamber music that has had such a range and quantity of advocates of the new amongst its ranks? For the ensemble with perhaps the 'fustiest' image of all, it's done pretty well in recent decades.

May post more later on why I think it still has plenty going for it, even now...

Elliott Carter's first two date in that period. Do they qualify?
Boulez's Livre pour quatuor is from 1948-9 but then he withdrew it.
Crumb's Black Angels comes from 1970... right at the end of the period.
How about Berio's quartet (1955-6) or Sincronie (1963-4)?
There's Ferneyhough's Sonatas for String Quartet coming in at 1967.

And indeed Heinz Holliger's first quartet of 1973. Nothing to compare with the flood of new work that's arrived since the mid-1970s though, I think. And, as martle says, the flood of new quartets building on the tradition (re)started by Arditti and Kronos.

I notice a new Skalkottas CD on the BIS new releases page which looks rather interesting, including a concerto for two violins, a concertino for two pianos and a piece for xylophone and orchestra. Do you know those works, auto?

Boris Tchaikovsky's music is especially attractive and lyrical, and I recommend it.  Two seasons ago we opened our annual Moscow Festival with his song-cycle "Signs Of The Zodiac" for high voice and string orchestra - lovely material indeed.

Although there was a hiatus of 100 years, the genre of the quartet or quintet of string instruments playing intricately-worked "academic" music had existed previously, of course - the enormous body of music for viol consort is testament to this tradition.

I've worked with the Arditti, Kairos, Pellegrini and Diotima quartets, and they're all as different as can be in terms of approach, and somewhat less different in terms of repertoire.

The Berlin-based Kairos, as far as I know, don't play "older" repertoire at all, the violinists share the leader's chair equally (unthinkable for the Arditti Quartet!) and they generally insist on playing from scores, which led to some spectacular upsets in the course of the first couple of performances of the quartet I wrote for them in 2002, because the lack of opportunity for pageturns in the score led to vast and precarious expanses of paper in front of each performer. Their performance of Giorgio Netti's quartet place is an astonishing thing. They tend to like discussing the music at length during rehearsals, as opposed to Irvine Arditti's more robustly practical attitude.

The Pellegrini and Diotima both regularly combine new works with "classical" repertoire. The Pellegrinis' CD of the Beethoven A minor and Nono's Fragmente-Stille is something I would recommend highly to all members - it's the recording of both works I come back to most often. Their recording of Feldman's String Quartet and Orchestra is also notable. Diotima, based in Paris, is the most recently-formed of these and the one whose work I know least. I was very impressed with them on a short acquaintance.

The Ardittis premiered my first two quartets (1988 and 1997) and, with ex-member Pace, my piano quintet in 2004. Their ensemble playing, as many members will know, is a wonder to behold in terms of discipline and coordination, and this seems to have varied little through all the changes in personnel the quartet has had. Not a moment is wasted in rehearsal, and all the material is painstakingly prepared in advance. They very seldom play pre-1900 music, with the Grosse Fuge being a glaring exception and a piece which in their performance seems to show an otherwise latent Xenakis "influence". In general I think their slightly astringent sound is better suited to (some kinds of) later music. Given that their ubiquity has made this sound the "default" for so much contemporary string quartet repertoire, it's too easy to then hear other quartets in say Ligeti or Xenakis and declare them eye-opening and revelatory when actually they're just different and less often encountered.

BZ, yes (and I suspect Stuart, hh and a few others...  )

And not to be outdone 

I've worked with the Brodsky (since college when they used to rehearse in the nude very intensively), the Bingham, the Smith, the Kreutzer, the Dante, the Balenescu, the Belcea and a few others. Reflecting on this, it's an extraordinary thing that so many quartets (an ensemble which hardly lacks regular repertoire) are so willing to stick their oar out and explore. I wonder where this comes from. A sense that innovation has often resulted from building on a more (rather than less) deeply rooted genre?

Not since I was an undergrad. And all three of those quartets are back in the draw now.
There are sketches for at least 4 new ones, but until I'm holding them in until I have the chance to work with a quartet.
Or until I get bored.