If you intend to use some effects that typically need to be in series, like compression, EQ, wah-wah, tremolo, distortion preamp, then you'll need the Eclipse in the preamp FX loop or in the g-system in the FX loop or post preamp, given a serial FX loop. If you only want to use typical parallel time domain fx (like I do) such as chorus, flanger, delays, pitch shifters, reverb, then using the Eclipse in parallel is a better choice.
The power amp should be placed at the end of the chain.
Yes, MIDI will do change presets on the Eventide if you connect the pedalboard MIDI OUT to Eclipse MIDI IN AND IF the G-System allows MIDI Program Changes to be sent out. Check its manual or TC support.
best
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