I have been using an EMG BQC preamp for some years ( at 18v ) with no problem. I also have a bypass switch installed. Switching between the BQC and passive mode has always kept the tone and volume level fairly equal with the controls set flat. For some reason this has changed and when I engage the preamp it becomes quieter and brighter. Perhaps it is an issue with the bass end of the preamp. The controls still work and it isn't the batteries as I have replaced them. Any thoughts?