Overall, have you played a good quality WAVE file through the internal speakers? Do they tend to 'color' the sound compared to a full range system? On the whole, I think we expect a bit much from rather small speakers in a VERY small enclosure.

One other possibility is, have you tried using a mixer's pre-amp to bring the mike up to line level? From what I can see at the Korg PA900 site's spec page is, the mike inputs are unbalanced:

Possibly a nice quality mike pre is needed? I have heard very good things about the Joe Meek ThreeQ (around $250 with a small footprint)