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If I want to control the overall volume of the M13 would the pre or post setting be best?

I assume if I used an expression petal for volume it would take up one of the FX's, is this true?

I have'nt purchased a volume petal as yet, so any recomendations from other posters for a quailty unit would be appreciated.

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Are you saying that the Volume module does NOT convert it's input/output to mono? That  would be WAY cool!

Thanks! That changes everything! 

If you're springing for the M13, and you don't need the send/return loop for something else, I think the logical placement would be to use the send/return loop (mono, or stereo if you want), so you can place the volume post-overdrive FX, but pre-delay and reverb FX for natural delay and reverb tails.  This way, you still have all 4 effects available!

Some folks like to place their volume first, but that works the same as the guitar volume control.  Placing it last after delay and reverb FX gives very unnatural sounding delay and reverb trails, particularly if you use the volume pedal artistically, as an expression pedal.  If you use the pedal just to switch between rhythm and solo levels, it doesn't matter so much.