Nice work! I particularly like the 2nd movement, with it's winding, pacing quality -- hauntingly beautiful.
Like Evan, i wondered a bit about how the perf-leg is used...(?) It sounds to me as though they are not often "leg"-ing! It may be that a little more phrasing of the line with slurs would bring out more of the legato quality. Strangely, I don't find the non-legato notes in the perf-leg instruments to be great for general use, and I try to avoid using them over too many consecutive non-legato transitions. For the clean, simple beauty of these parts, you might try making use of the progressive vibrato (pV) Violin, particularly in places like the high, sustained B, about 35 seconds into the piece. You might also mix in some senza vib samples in the Viola and Cello -- given the mood of the piece, I think the vibrato in the more ostinato-like parts could be toned down. The uniformity of vibrato on the mV instruments seems to push everybody into the foreground, not so much in terms of "the mix", but with regard to intention -- like everybody's got "solo" written above their part.
I'm also very much in support of using the perf-leg_gl instruments to get the sound of a distinct "shift". In spite of opinions to the contrary, many of the portamento samples are very subtle, and can really breathe life into the writing -- after all, players do *choose* to shift, at times (while at other times, they have no choice!). Picturing those choices in your head while writing can really help shape the line.
I'm also curious as to whether you took advantage of the release samples? In some of your writing they could really help -- I'm thinking particularly of the rhythmic unisons, where I seem to be hearing parallel, non-RS, "envelope"-like releases... (I could be way off on this, so ignore me if I'm wrong!). Here, the RS samples will help give a stronger impression of rebowing, particularly if you go in and edit your note endings to throw the sync off a little.
Regardless, it's great writing. And feel free to fire back if you've already tried most of the above mentioned techniques!
cheers,
J.