Will we be able to properly calibrate the TH555 with the METER module or will we need more specialized gear?
I believe & hope this will be possible with the METER module. And for those who don't have a meter module, the most important calibration should be doable with a tuner. The 'sine symmetry' and Triangle calibration can be done by ear if you don't have a scope/meter (because the sound is more important than the waveshape imo!)
However, i did not test it in practice yet. I calibrated the first breadboard prototype with a scope and the ableton 'tuner' plugin. The METER should have enough functionality to do both the Task. I will be able to give you An answer once the first pcb prototype is finished (just waiting for the very last component to arrive!).
I can add a few informations on the practical side, I designed the PCB to be tunable easily as you don't have to remove the module, simply need to unscrew the frontplate. The trimmer are labeled so you dont need to look for the manual, only a small screwdriver and you're done