In my experience (I used the ebtech with my Steavens Poundcake) the passive unit the Ebtech is, it sucks tone. What you want to make sure is you keep signal strength when using the loop exactly the same as if not using it, then the high end will all be there.
Best results will be hat with a GLab ALA-1, use the RACK effects loop of the shiva, and use the Glab to bring the level down before the pedals, then bring it back up after the pedals with the active internal preamp in the ALA-1. It's cheap and also gives you a 6m line from your amp to the board, with send and return both in one cable.