What sort of sub are you trying to use? The Ref 6 uses a differentially balanced circuit, so you can't connect its output to anything that has a common ground.
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What Curtis is saying is to find a sub that hooks up High Level
or Speaker level
dumping down the Tube preamp output impedance with a sub is a fool's errand and affects your great midrange
Many good subs can go speaker-level high out l Rel Vandersteens etc.
the DB1 is at its best with a surround sound processer in a home theatre
Cheers JohnnyR Audio Connection