Driver tube - I havn't seen anyone mention the Russian mil. spec. - 1578. I have 2 pre-amp driver tubes that are end game for me. The musical/emotional Tung Sol 6F8G RP, older version of the TS 6SN7 RP (some say, just a touch better). Then there is the 1578 - where the TS has wonderful warmth and musical detail, the 1578 is very transparent, detailed with great depth and a top end that goes on and on without a trace of harshness. In no way is it analytical.just very detailed and open. I use the TS 6F8G as driver and 2 1578 as o/put tubes in a hybrid head and power amp. This combo for me is end game, the two compliment each other so beautifully. I have used 3 x TS 6F8G for an overpowering emotional experience. Sadly neither of these tubes have been cheap for a long time.
Mostly the 1578 are sold out of Eastern Europe or the Russian Federation and sold in pairs or quads. Be VERY CAREFUL - if you can't see the multi-holed plates, they are a con. As pairs they normally sell @ $150. @$75 a tube they are up there with the best. At the moment I'm using one of these with a bog standard GE 6AS7GA. This o/put tube allows the character of the driver tube to shine on through. Many say the o/put tube dominates, this was so for me when I used a Svetlana which was too soft and creamy but the GE let's me hear the TS and the Russian.
I don't think that $75 is too much if you pair the Russian with something that may add a touch of warmth, certainly this is a sparkling open tube. If the Beyers could emulate the top end of the 1578, they would be no contest h/phones.