Hi, Piotr:

Composer should work very well with Garritan Personal Orchestra and I use MapleMidi myself with no problems.

Be sure that you have Garritan running prior to opening Composer, otherwise you can run into a conflict as both your virtual instrument and Composer try to use the same midi port. If your virtual instrument is already running, then Composer will skip the input port that it is using.

Good luck.

David