The Voladj feature of hijack should not cause any clipping or distortion unless you've set your EQ up so that some of its bands exceed 0db.

If you have your EQ set to exceed 0db, then there is a chance for certain passages of any music, voladjusted or not, to clip if you have the empeg's volume cranked up. Having voladj turned on simply means the quiet parts of the music are louder therefore increaing the chances that might happen.

Does it only happen when the empeg's volume is cranked really high, say, -10db or louder?