The string orchestra versions of the late quartets of Beethoven cast a warm and rich spell on the listener mainly because they are not re-orchestrations which we are faced with, but an enlargement, a strengthening of the voices and the timbres already present in the work.
This insight into the beautifully mysterious and sublime late string quartets of Beethoven was first brought forth by Serge Koussevitzky and then by his prodigy Leonard Bernstein. Since then we have had recordings by Andre Previn and as of late, Murray Perahia.
Weingartner's orchestration of Beethoven's late piano sonata, the Hammerklavier is however a different story.
Weingartner's orchestration uses the same orchestral forces as in the 5th symphony which in itself is quite problematic since Beethoven's orchestrations can really never be second guessed, as brilliantly illustrated by Bernstein in his orchestral presentation and discussion of Beethoven's sketches to the first movement of the 5th symphony itself.
Another aspect is the sense of scale (not to be confused with size) having to do with internal sense of proportions and relationships.. Having had seen reproductions of paintings of Salvador Dali in art books, I was quite surprised when I saw the originals to find out that some of the canvases were very small in size. I expected them to be huge. They were in a way; they had an enormous inner scale despite their small size. It is the same with Beethoven's late piano sonatas in particular the Hammerklavier. The late quartets are intimate pieces that can benefit from the extra dimension that the enlargement of the orchestral forces bring. The Hammerklavier however is already "symphonic" in scale. A precious little stone with the soul of a mountain. How can we add to that?