@Caveeagle Thanks for sharing your knowledge. I always wanted to add a Poseidon to my collection. I am thinking if I want to add a second stage I can just use a regular downstream second stage as the OPV.
I just use the older hoses with built in OPV or a OPV in one of the lp-ports. I don't feel rely comfortable with an other second as OPV but it would probably work just fine.
I have one Poseidon first stage with built in OPV (3970) that i considering to tune up, convert to yoke and use as my warm water single set.