Pete:
Is your SF-2 set to act as a pre-amp? There is an internal jumper that can be set for 0, 10dB or 20 dB gain. If you place it between your instrument and the amp, perhaps it is set for too much gain? I'd move it to the effects loop, as Jazzy suggests. And, of course, if you haven't already done so, check out the manual in the FAQ section.
Bill, tgo