After your amps are SE, do you use your Poseidon with some tubes removed?The Poseidon manual includes the following schematic
View attachment 133477
and states „It is possible and allowed to listen to one phase only if you use single ended amplifierr. Simply remove the tubes you don’t need.“ So my guess would be, that in balanced mode (and if the following component is really balanced), all tubes have to work properly, in single ended mode only one tube pair per phase per channel.
If yes, which tubes can be removed from the Poseidon for SE-only operation?
I use my Horizon SE with the right row of Pentodes removed not to burn down rare and precious tubes unneccessarily.