When using piezoelectric transducers, it’s always best to put a preamp immediately after the transducers, before the signal goes into anything else. If your instruments use batteries, they already have preamps in them so a preamp isn’t technically needed, but some people still like to use some high-end preamps or tone shapers to shape the tone. Again, these would go right after your piezoelectrics and before everything else. Both the Synth9 and the Mel9 want to see a nice, clean, strong, but not overdriven signal to track properly. Neither the Synth9 nor the Mel9, nor any of the other “9” series likes to see a modulated (phase, flange, tremolo, vibrato, ring, etc.) singal. They want it clean, well above noise, and not overdriven.

The order of the Synth and the Mel is not important. You can’t use them both at the same time anyway. If you want to get really technical about it, you can consider things like input and output impedance, and whether or not they have a buffered bypass, but the Synth9 and the Mel9 are identical in terms of impedance and buffering, so it really doesn’t make a difference which goes first.