Hi Thorben,
Do you typically use the KX 49 as your sound source? It sounds like (no pun intended) it's not set as the default MIDI Playback Device (yet!)
To set it as the playback device, (1) in Composer, go to Settings/MIDI Device Quick setup, and then (2) choose it from the drop list for MIDI Playback Device.
That should set your keyboard as the sound source, and avoid the GS Wavetable/ASIO issue.
ttfn,
Sherry