There's no "one size fits all" answer because the answer is context-dependent. The ProChannel was originally envisioned as a mixer channel strip. If there was no FX Rack, that would be the end of the story...the CE would go at the end of the PC. Although
usually it wouldn't make much difference if the CE was at the beginning or end, placing CE before Tube Saturation (distorting the CE) will provide a different effect than if placed afterward (CE processing distortion). Also, you have the same question as "EQ before or after compression?" where the answer is "it depends on what you want to do - compress the EQed sound, or EQ the compressed sound."
If you use the FX Rack mostly for time-based effects like reverb or delay, it makes sense that you would want to process the mixed sound. But, suppose you want 8 stages of EQ, so you use the ProChannel EQ and also insert an EQ into the FX Rack. Now the FX Rack is acting more like an extension of the "channel strip." In this case, you'd probably want the CE in the ProChannel to be post-FX Rack.
However, placing the ProChannel post-FX Rack has other implications. For example if you're compressing with the ProChannel, you might not want to compress the sound of the processors in the FX Rack. So then you have decide what your priorities are...
I would recommend placing the CE at the end of the ProChannel if you're treating the PC as a channel strip. OTOH if you're adding effects like delay or reverb in the PC, you might want to put the CE between the "virtual channel strip" and the "outboard time-based effects."
Just remember that ultimately, there are only some cases where there's going to be a significant difference between putting the CE at the end of the chain and elsewhere. Also, the CE will not be the determining factor in the appeal of a song. The CE
can add that extra little bit of coolness; you'll get most of the benefits of that if the CE is placed properly within the ProChannel.