pre amp blend system piezo/electret mic

Im looking for a diy projet or info amp how to build a
simple 2 chanel mixer with tone control and balanced outputs and unbalanced
maybe witha phase sw for the piezo input

I have build in a wm55 directional elevtret mic into my friends Martin guitar he already had active(buffered) Fishman piezo system i the guitar.
So im trying to get info for building some thing like this

Any advice is wellcome maybe thare is a better forum out there ?
for this kind of projects

best regards jc dk.