Hrm. (Yes, I'm opening up a somewhat old thread, and don't ask me what I regard as very old.) There's Reger's D minor op. posth. but that has bass ad libitum and probably mentions it in the title, and the obvious other example, Schoenberg op. 10, (etc. etc. ...) Since Sculthorpe's String quartet 13 'Island Dreaming' has an elaborated title but not one that mentions that it's a quintet for soprano and quartet or so I gather, how to classify it? (And does Leon Stein's fifth quartet, with soprano, mention the latter in the title? Admittedly, these are quintets.)