Audiohorizons preamp

I am using different Ortofon MM cartridges with my Audiohorizons preamp, but I'm having issues. One channel is noticably louder than the other and sibilance is very lispy. I've changed cables from RCA's to balanced but to no avail. My amp is a Mac MC300 and it mates well with a MAC C39. IC's are Cardas and speaker cables are anti-cables, while my turntable is a Technics SL1200 MKV. The tubes are Electro Harmonix in the phono stage on the AH. The line stage works fine, it's just the phono section. I've sent it back to Cali, but Joseph Chow can't find a problem. Any suggestions are welcomed.