The idea of an FX loop is to insert the effects between the preamp and the power amp section of an amp to prevent the effects from distorting. The more gain you use the more audible the benefit becomes. What differs from amp to amp though it the return point of the loop. Some amps have the return after the master volume control of the amp. If you insert additional noise from an effects device or use a cheap cable, the additional noise is amplified with the full power amp output. Very nasty.
I now know about this loopy stuff! Thank You!!