I have an Adcom GTP-450 stereo-only preamp with an "External Processor Loop" L/R in/out. It is not discussed in the manual. I assume it is a signal-level loop for adding an EQ or something similar? Is it a constant level signal or is it controlled by the volume setting? If it is inline with the primary signal path, I can't understand how it can be an open loop by default - ie. the ins/outs are not currently connected but the preamp works fine. Any clarification would be appreciated. Thanks.