Robert,
Some of the laternative tubes I'm considering for my s.e.x. 2.0 are the el84 and the 6v6 (both in triode mode), the 12b4a (very nice clean tube, but would need a driver stage as it is only a single triode in each bottle), and the 6bm8 -- like the 6dn7, a gain and power tube (pentode, so again, triode strapped), which is also a really nice sounding tube -- one of the few designed specifically for audio use.
Not sure which I'll go with, but also some of the related tubes could be considered -- 6k, 6y6 as well, also some decent sounding tubes. Of course, all but the 12b4a are pentodes, so they will have a characteristic sound somewhat different than the triodes.
HTH,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)