Ok.
But... always.. after a certain amount of time the distortion will return. Also, my system sounds terrible under Scheme 4 (scheme 3 is the best).
Suppose this still happens (but maybe later now), try to learn how the graph of your CPUs look like when all is fresh (reboot) and playback commences. Then, at the stage things are starting to distort, look at the CPU graphs again and see if it has changed.
If so, it should be the (Weiss) driver doing it, which you -with some experience- can see by comparing it with *no* playback is going on. Also notice that for a higher buffer size of 2048 this will be far less noticeable than for a buffer size of e.g. 64.
If you see anomalies here, you may check the Weiss website for newer drivers. Or older ...
Good Firewire drivers are like an impossibility ...