@Siwi and Gardner: Consumed and understood, and I appreciate what you have to say.
It occurs to me now that perhaps my attitude was a little too strident in my earlier comment, which comes of having heard my work ruined to some degree on more than one occasion. Let me step back from that a bit and clarify some things.
Being primarily an 18th Century Historicist (Classical and Baroque), I actually conceive and score my music for period instruments - in this case, natural horns. However, this doesn't mean I'm going to turn down a performance by a modern orchestra, even if it wouldn't be my first choice; after all, modern orchestras play 18th Century music all the time. Even so, I don't approach my work on the assumption that only modern players will be performing it, as my preference would invariably be a performance on period instruments. I want to make that very clear.
Next, I should mention that I have been asked to make concessionary re-writes in my symphonic works to accommodate modern horn players, and these revisions I did make, however begrudgingly; yet despite these accommodations, there were still problems in performance that in my opinion should not have been an issue, because these same players would have performed similarly-scored music by Mozart or Haydn or Beethoven with as much technical prowess as they possessed, resulting in far fewer errors. I gave my horn players their parts well in advance of the first rehearsal, so they knew what they were in for. Why, then, was not the same effort and preparation put into my work as would have been put into that of the aforementioned masters, whose techniques I follow practically to the letter?
In light of this, the issue then becomes, for me, one of professional integrity and respect for the composer - and therefore, it's personal. Whether my work compares to the masters in quality is immaterial, particularly when I have gone to the trouble of re-writing a part I should not have had to re-write, for no other reason than to make the players' lives easier. Siwi's analogy that my horn writing is the equivalent of writing a violin part for the viola doesn't really apply here, because in fact, horn players can and do play this kind of music all the time, whether it's difficult or not. In my historicist work I operate as an 18th Century composer (were I to do otherwise, the authenticity I demand of myself would be compromised), and as I am in no way equivocal about this, I have every reasonable expectation of being interpreted as one.
As for possible solutions to the issue: if a player thinks that it would be easier to play something I've written on an instrument other than a modern horn in F (descant horn was mentioned), then more power to him. Whatever will solve the problem is fine with me, though I have yet to have someone even bother to suggest such a solution; the players I deal with seem to prefer complaining to solving the problem, which is ultimately theirs to solve. But I'm not going to change the way I score, which is exactly the way my models scored in their day, and their pieces are still played by what is in standard use today, regardless of what they were actually written for.
Do you see where I am coming from here?
Another thing I'd like to address, for what it's worth: while the parts written for natural horns and trumpets in the 18th Century may seem relatively undemanding compared to Mahler (to quote Siwi's example), let's not sell the musicians who had to play them short, nor underestimate their stamina. A fact that is not generally known is that horn and trumpet players were formerly trained on horseback, their primary purpose having been to serve as signal instruments for hunting and military use. Hence these players were rather overqualified for sitting comfortably in an orchestra, no matter what they were asked to play. In light of that, I don't think it's accurate to assume that they would not have been the equal of any modern player in sheer stamina. Quite the contrary, in fact, since I don't believe modern brass players are typically trained on galloping horses anymore.
Put yet another way: if 18th Century composers' experience of brass players had been that they were not capable of playing the music they wrote for them, would they have continued writing it that way? Probably not. But the fact is, they did continue in the same vein (though music got less demanding for trumpeters as clarino technique went out of favour), and in the case of Beethoven and his contemporaries, the music written for the natural horn in particular only got more challenging. It's therefore logical to assume that the players were more than capable of accurately executing what was written for them most of the time. These same parts are played by modern players today with a reasonable degree of accuracy, difficult or not. Therefore, why should I be obliged to operate any differently than my models did in this regard? As I see it, the answer is: there is no defensible reason why I should, and I won't.
All this notwithstanding, in light of my real-world experience, if I were ever asked to write a Classical piece specifically to be played by a modern orchestra, I might think twice before doing so in any key higher than F; but that is the only concession I am inclined to make.
I hope that clarifies my position somewhat.