When powering pedalsHIELD MEGA from USB, the noise generated in the power supply is quite strong and could degrade the audio signal, form my experience this is the best way to power the system in order to improve the sound quality:
1- Best Solution: Use an External 9 -12V DC adapter 2.1mm center-positive plug. There is more info about which adapter to use in the Arduino website ( playground.arduino.cc/Learning/WhatAdapter )
2- Intermediate Solution - USB wall adapter, depending on the brand some may sound better than others:
3- Worst Solution - use a computer/laptop as a source of energy. It is good an quick to load effects and verify that they work, but once you are sure, use a wall adapter.