Bad monkey noise

Are you using a filtered/regulated power supply? Do you still get the noise if you use a battery?

 

Dirt pedals in general typically work better going into the input of the amp rather than in the effects loop. I quite like using a clean boost in the effects loop though.

 

I'm no expert but I've read that depending on the amp, the "preamp out" can be a "line level" signal. There is a significant difference in a line level signal vs. the "instrument level" signal that comes from your guitar. That right there "could" be the issue. I know some pedals are designed to handle line level (like some high end delays, etc that sound best when through a preamp out or effects loop out). But a lot of effects are not designed with that purpose, they are designed to accept instrument level.

FWIW I tried several of my OD pedals through the effects loop in my amp and found the sound quality to be terrible, there was a loss of available gain/volume as well as loss of clarity. YMMV. A lot of people have better success with a delay/reverb in a loop.

 

The effects loop allows you to use your effects in the "Right" order. After the input, your signal goes through a gain stage and through the speaker. The effects loop is in between the gain stage and the speaker which allows you to put pedals that usually work better after a gain stage in a spot were they "should" go.