The other WK's have outputs[WK3500, WK3700].

I use the WK300 thru a 1,800 watt sound system, with no fuzz,, Try keeping the volume on the WK at 12 O Clock or less,..
Adjust the volume at the sound system..

You may be hearing some effects..some of the effects will distort..Try not to use the effects that may cause the problem..