Fkalich, yeah, that happens for me, too. It sounds as if the amp is acting like a hi-pass filter and the octaver is producing sounds like it can't reproduce. Around the A, maybe the G on the E-string, it's moving some serious air, but just drops out any lower than that. I think the octaver is adding a note 2-oct down, where the note I use with the diatonic or chromatic is 1-oct down, which would explain some of this behavior… the fundamental 2-oct down from the low-E in standard tuning (a440) is 20.6Hz. Not many systems can reproduce that… and the PF may be filtering it out to prevent us from accidentally blowing up our amps.
Glad you can get some good sounds with it in front of the amp… I've never had any luck there, either. It's not that the ocatver is a bad effect, just not quite my taste. I prefer the H910 for sub-octaves, but that's just me. What settings are you using in front of the amp?