Output tube types - need some opinions

Depending on your power requirements, any pushpull from a 300B, 2a3 or 45 tube would be a worthwhile project (I particularly like the 45, but, it is very low in output).  Spend all that you reasonably can on the output transformers.  I like vintage transformers such as Accrosound TO 330 or Chicago transformers.

For pentode/tetrode tubes, I am a big fan of 6L6 and KT66 tube.  Even more exotic would be the Western Electric tube that is close to the 6L6--350B, which is a really nice sounding, albeit expensive, tube.  My own pushpull amp runs the Western Electric 349 tube (very low output, even more expensive than a 350B).  Again, the output transformer is very important.  The vintage ones I like were those from RCA 6L6 amps and from Northern Electric amps.  The Western Electric 171C transformer, despite its limited bandwidth, is extraordinarily nice sounding with these tube types, but it is quite hard to and pricey.  

I have not heard any amp built with KT120 or KT150 tubes that I really liked--somewhat hard and brittle sounding.  I feel the same way about most KT88 amps as well.  I actually prefer most "budget" El84 amps to those using the more powerful alternatives.

By the way, I really like your build projects posted on the internet.  I am particularly impressed with the way you make each project your own, in terms of design and execution, and the build itself is meticulous, and beautiful in appearance.

A classic EL-34 is a good starting place, then maybe retune for 6L6. 

Thanks for the feedback and ideas. The 300B is one of the tubes that was on my list to consider. My only reservation was the limited power and, to some extent, the cost of the better tubes. On the other hand, it would give me an excuse to try a set of Takatsukis to go with my WE reissues 😀.

@larryi - thanks for the complements. 

I have been researching various brands of transformers. Are the Tango transformers still being made, or are these only available used? Any feedback on Monolith Magnetics? I've also been looking at Toroidy, but I haven't seen many people using toroidal transformers for OPTs. 

I don't think Tango is still made; a shame because they are reputed to be quite good.  A local dealer that also makes its own amplifiers likes Hammond for custom-made transformers (they will wind to your specifications).  I think he also likes Electra-Print.  Another friend likes the Japanese Hashimoto transformers.  I don't know anything about Monolith Magnetics, except that their stuff looks good.

  Are you on the  Alpe d'Huez ? No wonder you want more HEAD ROOM 😎. Much respect to you for your choice of rides and your choice of amps. Looking forward to hearing about your built. Ride hard, Mike B.