The Ref 6 uses 6H30P tubes, which is a Russian tube that has no substitute. The older DR version is considered to have better sound, but they are very expensive and rare. So unless you're willing to pay the price for the 6H30P-DR, there's no tube rolling for the Ref 6, except for the 6550, of course.
ARC Ref 6 tube replacement
Looking to replace stock tubes in my Ref 6. Anyone replaced their stock tubes? I also heard that that the NOS Tung Sol 6550 is an excellent tube for the power supply. Anyone have knowledge of this?