Hi Newbee - Mozart and Haydn indeed did begin writing works in which all of the movements were unified by motivic development. Mozart's last three great symphonies, and also the Prague; Haydn picked up on this further after Mozart's death with the London symphonies. This was not a new thing with Beethoven, though he did it with much smaller motivic units than his predecessors - this was the main difference, as well as his expansion of the symphonic form. Beethoven's Romanticism is often greatly exaggerated - formally speaking, he is a Classical composer through and through. This is even the case with Schubert, when he writes in sonata form in the symphonies, chamber music, etc. It is in the song form where Schubert begins to be Romantic. The Romantic movement in music does not really start until their successors - Chopin, Schumann and Liszt in particular. These are the first composers who are completely Romantic in outlook and form. The late Charles Rosen wrote a great book on this very subject, which I happened to just finish reading a couple of weeks ago.
As far as the horn in Wagner's day - it was actually very little different from our modern horns, except they were mostly single horns, and still used some crooks, as opposed to the modern "double" horn which does not require crooks. But the valves on the horns in Wagner's day were very little different from our own. The Vienna Philharmonic still uses that type of instrument to this day, by the way. I am a big fan, and keep threatening to buy a Vienna one of these days and use it in my orchestra. As a low horn player, I could probably get away with it in many circumstances. If you search the Vienna Horns, they have recorded a couple of great discs. Those instruments are essentially unchanged from Wagner's day.
Also, the size of the orchestra Wagner ideally liked for his works was indeed the equivalent of the modern large symphony orchestra, though he often had to make do with less earlier in his career - but this was not by his choice. In fact, Wagner loved to triple the winds when performing Beethoven, making the orchestra even larger, something which is not done anymore generally (though again, it is sometimes done in Vienna in certain cases, for instance the third movement of the 6th). Mahler also liked to do that.
As far as the horn in Wagner's day - it was actually very little different from our modern horns, except they were mostly single horns, and still used some crooks, as opposed to the modern "double" horn which does not require crooks. But the valves on the horns in Wagner's day were very little different from our own. The Vienna Philharmonic still uses that type of instrument to this day, by the way. I am a big fan, and keep threatening to buy a Vienna one of these days and use it in my orchestra. As a low horn player, I could probably get away with it in many circumstances. If you search the Vienna Horns, they have recorded a couple of great discs. Those instruments are essentially unchanged from Wagner's day.
Also, the size of the orchestra Wagner ideally liked for his works was indeed the equivalent of the modern large symphony orchestra, though he often had to make do with less earlier in his career - but this was not by his choice. In fact, Wagner loved to triple the winds when performing Beethoven, making the orchestra even larger, something which is not done anymore generally (though again, it is sometimes done in Vienna in certain cases, for instance the third movement of the 6th). Mahler also liked to do that.