  ::::Not necessary. I must have been half asleep when I read your edit - it is perfectly clear and it was my reading that was blurry.  ::::Actually, I believe that Boccherini's piano quintets preceded Dussek's (http://imslp.org/wiki/6_Piano_Quintets,_G.413-418_%28Op.57%29_%28Boccherini,_Luigi%29). But Boccherini's quintets (and, I am guessing, Dussek's as well - I can't find a score for them) are not anything like Schumann's. There is no real interplay between the quartet and the keyboard (which, actually, might have been a cembalo in Boccherini's thought), the first violin doubling the right hand, the cello the bass line, almost throughout. So I think it is fair to say that Schumann invented the form, at least as we know it today.   