Well I solved my own problem following prudent steps such as floating solder joints in the high filter section starting with switches and pots. I found a solder bridge on the gain pot for the high filter and removed it. All others were re-floated on frequency and switches just to make sure. Retested and distortion is gone but the filter is acting strange so back to the
"0db gain pot calibration" and after rechecking all frequencies the high was out so I tweaked it back in to spec and re-installed it into the 500 chassis. I ran tests on kick, snare, acoustic guitar and some vocal tracks. I'm really liking the sound of the eq on all sources particularly the acoustic guitar and vocals. All adjustments were rather broad and small but added a nice pleasing shape and sound without adding unwanted character that some of my other EQs would add. All around I'm pretty darn happy with the results. Great work Yann!