Question I tried my SCF pedal in the parallel FX loop of my amp. And there was a very audible loss in sound, turning the amp's loop's mix control from dry to wet, even when the SCF is bypassed. What could be wrong here

The SCF is not suitable for parallel loops since the dry signal is also passed trough the SCF. This causes phase problems and will ruin the sound. You should use the SCF in an serial routing but it might not have enough headroom for use between pre- and poweramp where most FX loops are positioned (depending of amp and loop type).