This is the critical part of the system as I need to "merge" the Stereo signal into Mono. As the Amp Camp Amp is a power amp, I need a Passive Volume Control also.
Important : Simply using a Y-splitter in reverse will not work. It'll cause leakage and distortion. Searching the web, the right way to do in summing is to add a 1K ohms resistor into each path before merge them.
So I put together a box for that. All parts from PCX (https://www.partsconnexion.com). The ALPS potentiometer is a great option without breaking the bank. Smooth, zero-noise.
Outcome : a simple and high quality Volume Control within arm reach.