It sound like you may have been having a level and/or impedance issue with the PF in the loop. We haven't tested with Carvin amps, but the amps we have tested with have a pretty large range of levels in the loop. In fact, several amps tend to have lower levels in the loop than you'll get out of a medium to hot pickup. It's also true that the octaver effect locks in to the fundamental frequency of what you're playing and if you're playing through a bright amp or just have the lows turned down the octaver might not have enough low end to lock on to the fundamental of a low E. The other possible issue is if you have a high gain distorted signal into the octaver effect as that can hide the fundamental. While most of the shifters in the PF probably sound better after distortion (though they can usually go pre for a different sound) the octaver probably wants to be before the amp.