EDIT: how could i forget? sorabji's 1st quintet. id have to listen to it a few more times to comment.
Which performance of it do you have? (none is yet commercially available). So far, only three ensembles have performed it in public; Christopher Berg and a group of freelance string players in New York in December 1998 (coincidentally on the same day and in the same city as one of the early performances of Elliott Carter's Piano Quintet [now there's one!]), Frank Peters and Quatour Danel in the Netherlands and Belgium in 2003 and Jean-Jacques Schmid and a freelance group of string players in Bern also in 2003.
Then, of course, there's Sorabji's Second Piano Quintet, but this has not yet been performed (and it's a really big one that occupies 432 pages in the ms. and 459 in the recent typeset edition) and is hardly "modern", of course, dating as it does from 1933. Although it's also hardly "modern", let's not forget Ornstein's Piano Quintet of 1927 (which Marc-André Hamelin will be performing next year with the Arditti Quartet). Thomas Adès has written one. So has James Dillon (called "The Soadie Waste"). There must be plenty of others!
Best,
Alistair