Tube amps - what 3 things…

I agree with the previous comment on transformers being the most important component.

Until they aren't. Tube amplifiers with output transformers, regardless of how good they are wound are not as transparent as their OTL or direct drive counterparts (given a level playing field). I've have all three design types and it's pretty clear what an output transformer adds to the mix. Getting rid of the capacitor at the output is also a benefit.

I have a selection of amps including Naim (solid state), Vincent pre / power (Hybrid) and recent Cayin (tubes). Currently despite being the cheapest the Cayin clearly wins in my system (but with EL34 tubes not KT88's). Three lessons, 1, forget the technology listen to the amplifier. 2, some speakers suit some amplifiers, some do not, listen to speakers first then buy an amplifier that suits them. 3, don't be hung up on price, I auditioned at least 20 tube amplifiers before buying I am perfectly happy with the cheapest, by a long, long way. Features, don't pay for ones you don't need, they add cost and complication. Think cars with multiple 'drive' settings, the 1st month you try them all, 95% will never change them after that. 

My Firebottle is from late 2016 (it has its birthday on the bottom with Had's signature) I and  bought it in early 2017 for 1100 bucks or so. I'll never sell it.

Hidaleeho.  Having computer troubles, so just seeing the status of this thread.  Good exchange of info...good.  Ralph, I'm sure that I'm wrong because I'm even more sure that you're right.

Got some 8 Henry chokes on the power supplies of the amps at chez xeno, and wow, they're lurkers.  Carry on...

Cathode bias is not a good thing in input and driver stages, not just in the output stage.

I tried one solution in my DIY SET 300B amp. There is 6sn7 input tube and 6F6G in triode mode driver tube with coupling capacitors between stages. The solution is very big, around 100000uF cathode capacitance in the input and in the driver tube. It gave a huge improvement in everything: bass, soundstage, separation, high resolution.
It is a pity the same solution can't be used in the output tube cathode, because of high current start-up issues. The Maximum capacitor I can put in my 300B cathode is 5000uF. It improved sound but it can't exchange fixed bias in terms of SQ.