Hello Hans,
Just had a listen to your demo, nice clean sound, and have a couple of suggestions for you:
1. Spatiality. Your mix (on headphones) is largely centred, except for the piano which has a broad stereo image. You will get better and clearer results if you pan your instruments in the way they would be arrayed on a real stage, perhaps horns to the centre-right, brass and timpani to the left, percussion spread out, etc. Garritan has lovely pictures of all this, but of course you (as is a real conductor!) are free to dispose your forces as would an army General. There are standard patterns, however, and there is a reason for that. One convention or tradition which has fallen out of favour (unjustly) in the last fifty years is the placement of first and second violins left-right on either side of the conductor. Pretty much all classical and romantic music was composed with this architecture in mind—and it actually sounds better! Meanwhile, reduce the spread of the piano so it appears proportional to the much larger orchestra (I know many engineers make the piano huge and across the entire soundstage, but they really shouldn't). I have asked elsewhere on the forum if it is possible to get a proper 'audience perspective' from Pianoteq, meaning the high strings will appear more to the left rather than the right, but am yet to resolve that.
2. Depth. This is simply a function of how we perceive the balance of 'direct' sound to 'reflected' sound. Close your eyes, you can tell not only where a sound is coming from, but also how far away the soundsource is. So to take advantage of this, you need to 'dial in' appropriate levels of reverberation (preferably impulse response like Logic's Space Designer or Altiverb) for the different distances from the 'microphones' or 'audience'. So, put some on the violins, maybe a little more for the violas & cellos (depending on your stage arrangement), then more for the winds (front row flutes and oboes, back row [a little more reverb if you're fussy] clarinets and bassoons), then add more for the brass and horns, and finally you have the percussion at the back. But remember that engineers will also use 'spot' mikes to get more 'direct' sound from (a good example) the timpani. This, then, is ultimately a matter of taste and experience.
3. I use Logic 7 and am familiar with the EXS instruments. Orchestrally, they are not really competitive with dedicated orchestral libraries, of which Garritan's GPO must be the most comprehensive at that sort of price point. The instrument patches are set up to be nicely expressive using the MOD wheel for volume/tone and the sustain pedal for legato articulations (it doesn't sustain like a piano, but blends the join between notes quite effectively). There are many other worthy libraries you can check out; you'll have to acquire something like this if you want to make the finished product believable. Then again, back in the late 80's I did some scores using old Roland PCM modules, a Sound Canvas and various synths, for a University Ballet Performance. People asked which orchestra had made the recording? So I think points 1 & 2 above are just as important as number 3.
Happy to answer any other questions on this subject you may have,
Cheers,
Stephen.