I had a pair of Premier 12 monos for several years and after a bit of experimenting I concluded they sound best with the GE6550A they originally came with. Barring 2 quads of NOS black plate Tungsols dropping into your lap, my next choice would be the SED ("winged C") 6550C. (Svetlana 6550Cs have the 'S' logo, and are not the same as the SED.)
Rolling the driver tubes in the 12s can have as much impact as swapping the outputs. With the GE6550As, a single RCA triple mica black plate 5751 is a nice improvement over stock and RCA 6FQ7/6CG7 clear-tops also work really nice, or late 50s Sylvania 6CG7 are even nicer.
Rolling the driver tubes in the 12s can have as much impact as swapping the outputs. With the GE6550As, a single RCA triple mica black plate 5751 is a nice improvement over stock and RCA 6FQ7/6CG7 clear-tops also work really nice, or late 50s Sylvania 6CG7 are even nicer.