Rajan, thanks, those old threads were helpful.
I'm finding that both the 12AT7 and the 6SN7 are important in different ways. It's hard to characterize, since both tubes play a role in all aspects, but from my preliminary attempts (four brands of 12 AT7, and four brands of 6SN7, more inbound) the 12AT7 seems to be the primary tube in terms of detail and "warmth", while the 6SN7 appears to be the primary tube in terms of frequency response (more importantly, the shape of the frequency response curve). Both play a role in dynamics. No matter what the 6SN7 is, dynamics will be restrained if a JAN Philips is the 12AT7. Put a JAN Philips at the 6SN7 as well, and the sound will be incredibly smooth and silky, but completely incapable of reproducing rock...just not enough punch. Switch out the 12AT7 to a Raytheon, Telefunken or an Amperex (Raytheon has some veiling, Telefunken is classic "tube warmth" Amperex is the most neutral so far), and all of a sudden the 6SN7 can let go...but a JAN Philips GTA won't let go as much as a Raytheon or a Sylvania GTB. The latter tubes have aberations in the frequency response to the point where they won't be my permanent ones, although earlier versions of the Sylvania's might do it (The GTB's seem to have a dip in the midrange...odd). I've also used a China Metal Base from Upscale, but there are better tubes out there. More on this as the tubes keep rolling...
