ARC REF6 and B@W DB2

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.

Sub is https://www.bowers-wilkins-custom-id.com/wp-content/uploads/2017/01/DB2D-Information-Sheet.pdf 

sorry, i'm beginner, unfortunately i can't understand what is mean common ground. thank you for you help.

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

Jensen Transformers SP-2SX Stereo Speaker to Line Audio Converter/Isolator

Now SUB is working!!!!!!