The effects loop is after the master volume, so the amplifier works at 100% for anything that's fed into the return. If you have any noisy pedals, the noise floor will be as if the amp is all the way up. This is not a problem when playing at loud levels, but when playing quiet it's quite hissy. For this reason I run an active volume (Morley optical style) pedal at the end of my effects loop and use it as the master volume of the amp. The actual amp's master can then be turned up to minimize the noise floor by having a higher signal into the pedal chain.