Brownsfan, interesting that you mention Mahler. Just a few days ago I had the pleasure of taking part in a performance of Mahler's 8th symphony. As you know, it is scored not only for large orchestra, but also for large choral ensemble. The subject of blend (or not) vs clarity of individual line was one that was discussed at length during rehearsal. This article was used as one of the references on the subject. You may find it interesting:
http://books.google.com/books?id=xE9PQzkLOZkC&pg=PA15&lpg=PA15&dq=mahler+and+instrumental+blend&source=bl&ots=ys1IHesuQS&sig=UxYCOhfUuHt2u_neOhzWxrb9IHE&hl=en&sa=X&ei=LrDfUJqUCqq00QGtooHIDA&ved=0CD0Q6AEwBDgU#v=onepage&q=mahler%20and%20instrumental%20blend&f=false
http://books.google.com/books?id=xE9PQzkLOZkC&pg=PA15&lpg=PA15&dq=mahler+and+instrumental+blend&source=bl&ots=ys1IHesuQS&sig=UxYCOhfUuHt2u_neOhzWxrb9IHE&hl=en&sa=X&ei=LrDfUJqUCqq00QGtooHIDA&ved=0CD0Q6AEwBDgU#v=onepage&q=mahler%20and%20instrumental%20blend&f=false