There are many factors, as Diki said, the room acoustics and adapting styles etc. is important.
Concerning your questions regarding audio card and preamp, IMO those factors do not make sense in your case. What role should an audio card play? You use the sound processor of your keyboard, so that's it. A preamp doesn't help if you use active powered monitors like the M-Audio BX5 (or Genelec, or an active consumer sound system like the Bose Sounddock 10). The reviews on the BX5 at Thomann seem positive. I would try to adjust the keyboard equalizer; as one customer writes, the BX 5 could use enhanced high frequencies so try that.