Thanks for that, Barry. It was pretty interesting. I remember fond times in high school having similar discussions about trumpets, especially the post horn solo. The ballerina solo from Petrushka was also heavily debated; should you use C cornet, regular C, or even D trumpet. Even now there seems to be small divide growing between playing C and Bb in the orchestra. And, as always the rotary and piston valve argument.
My school had a very nice old instrument collections, sadly no posthorn. We did have a Serpent and an Ophicliede. One of the best 'sounds' for the posthorn we came up with was to take a Crown Royal bag and put it over the bell of a Schilke C Cornet. It was a wonderfully deep, dark and mellow sound.
I tend to agree with your sentiment that Mahler would not have cared what physical instrument is being used. He was more concerned about the tone and color of the sound. Get that however you can. One thing I've always wondered is would Mahler have changed his scores if he heard a modern orchestra play his music?
