I downloaded a self-made midi-file to my 6100. It uses:

Pad2 (90) using 1 notes at one time (melody)
Tenor Sax (67) using 2 notes at one time
= 3 notes simultaneously

Since the 6100 is able to play 4 notes simultaneously this shouldn't be a problem. It's all right when you play the Pad2 or the Tenor Sax separatly. But played together, you get a cracking sound on the beginning of the melody-notes. What could be wrong?