With my limited edyamakashun, I will throw out a hypothesis and say more than likely. If it is a parallel loop, I really believe you would still hear the signal routed through the rest of the amplifier. And since 99% of loops are parallel, Robert's yer auntie's husband. Now a series loop is a horse of a different color.
I know I have a rack tuner plugged into my amp loop, with just a short cable from the amp FX send to the tuner, and nada from the tuner back to the FX return. It's been working fine for a couple of years now. So my hypothesis is based on some experience.