Hey Nathan, Never dove a poseidon, but parts kits are costly, and tough to find reps to service them. I use an assortment of Scubapro First and second stages. I use a Mk 17 (environmentally sealed balanced diaphram), and love it (forget if you use AGA`s or other FFM`s, or second stages). I have a G250HP, but would definately look at the new G250 V(intage)with a metal barrel for heat transfer compared to the plastic of the HP. Seperate PM sent on pre and post dive procedures to minimize freeze-ups. All the Best, John