If you want it to sound like an Ampeg reverb... design it like an Ampeg reverb.
To do it otherwise... you may not get the results you're looking for.
"one gain stage ->cathode follower/tone stack->LTPI"
Not very 18Wattish, but I think I get your drift.
Without some closer looking in to, I would say its not the best
place to pull you send signal from, but it may be your only choice. Off hand it just seems there should be another gain stage right after that tone stack, and that would be the place to pull a cap coupled reverb signal from, right off the plate.
Sounds like your in the planning stages still.... keep thinking things through and ask questions, even when you think you're absolutely right.
help to get your ideas across clearly, what I'm imagining the topography of your design looks like, may be way off base.