Tube amps - what 3 things…

The most important features depend largely on the buyers attitude and expectations regarding components.  If you are someone who is bothered by having to maintain/fix gear, or are bothered by the fact that tubes start to go bad from the moment they are turned on, then tubes are not for you.  To the extent you have some tolerance for the nature of tube gear, but, you are still concerned about tubes wearing out, that might affect the kind of gear you should buy.  The type of tube chosen, and how hard the particular amp drives those tubes, are BIG determinants of how long tubes will last.  There are amps that will run for many decades without the tubes having to be changed, others might need new tubes every couple of years (or earlier).   Tube amps tend to be more noisy than solid state--hiss or hum or intermittent crackling, etc.--you need to factor in your personal tolerance in deciding what to buy because some otherwise great sounding gear might be prone to be noisy.

For all kinds of gear, not just tube amps, decide what features are necessary.  To me, remote control of volume and some form of balance control is essential.  Given the nature of tube gear, I would think that small channel imbalances are inevitable and the ability to compensate with a balance control is a must.

The world of tube sound is vast and very variable--much more so than the sound of good solid state.  You need to listen to enough gear to determine what fits your ear.  Personally, I don't think there is only one type of tube topology (e.g., single-ended triode) that is clearly superior to another, but, there are some listeners that have a clear preference.  I have heard versions of pushpull pentode/tetrode amps, single-ended triode and output transformerless amps that sound terrific, often in quite different ways.  I think the type of tube, and the brand of tubes matter too, but again, these are personal preferences so the only recommendation is to hear plenty of different examples to develop your own idea of what to buy.  Finding a friend and/or a friendly dealer that can help you sort out options would be a big help.  If you attend a big audio show, you can also taste different gear or find people that can help you make the right choice.

I too like EL84 and 6L6 tubes.  The KT66 is pretty similar in sound to the 6L6.  The EL84 must be an easy tube for builders to work with (probably allows for use of cheaper output transformers) because that is where you most often see it being used, but, I like the lively and punchy sound that tube delivers without being harsh.  The 6L6/KT66 are terrific sounding tubes--rich, yet plenty of clarity and good amps running those tubes sound "big" (enveloping soundstage and sense of weight and scale). 

Of the small triode tubes, I like 2a3 and 45 the most.  But, of course they do have limited output and cannot be used everywhere.  

I don't know about the tubes that are used in the output transformerless (OTL) amps I've heard and liked.  Unfortunately, there aren't that many makers of this type of amp because many tend to be unreliable.  That left Atmasphere, a very reliable brand, to this field.  The Berning amps (including the Linear Tube Audio) are sort of variants of the OTL (they are interesting, but for my taste, they are not voiced the way I like tube amps).  Anyone interested in tubes MUST, at some point hear an OTL amp--for some it is like a religious experience--the dynamics and liveliness can be shocking.

With tube amps, more so than solid state, you really have to hear what you are considering for purchase because of the wide range of sound.  While I am a tube electronics person, I find that there are plenty of tube amps that I would not want and I would get a solid state amp instead.  This is particularly the case with high-powered tube amps.  If I really need that much juice (I don't because I don't listen at high volume in a large room), I would consider solid state instead.