Tube amps - what 3 things…

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.  

No more noisy tubes, better bass extension and tone with SS
No more headaches getting good vintage tubes, no more need for a finicky vintage tube tester, no wasting time tube "rolling", no more worries about Russian supply or Chinese garbage, etc.

This comment is the opposite of my experience.

My tubes are quiet.
My tube amps do a fine job regarding bass with my well-matched speakers.
Vintage tubes are not necessary. Very good tubes are in easy supply and not expensive.
No tube tester has ever been needed.
Tube rolling is for fun and not needed.

As a guitarist, I’m probably overly attached to tubes.  I own over 30 tube guitar amps.  I also own several solid state - digital and analog- guitar amps.  They’re inferior - except for portability and consistency. Tonally, there’s no comparison.  Tubes rule. 

I also own McIntosh, Prima Luna, Manley and several SS  Marantz - Ruby, Reference and a 2230 - hi fi stuff.  

The Manley Stingray  is EL84 based.  In guitar world, EL84 is known to be easier to drive, suitable for low wattage amps and possess a desirable chime at lower volumes.  The Manley fits the bill.   It sounds great, gets to the sweet spot without cranking it up but the bass response is weak.  EL84 based systems require efficient speakers.  Still, a great sounding integrated from Manley.  They build amazing stuff.    But I would probably pass on an another EL84 amp.  

6L6 is my favorite in both hi-fi and guitar sound.  It’s clean, robust, powerful and has a ton of headroom before it gets flubby.  Tubes are easy to find.  The McIntosh integrated is 6L6 as is the Prima Luna separate.   My main guitar amp, a Two Rock, uses 6L6.  In guitar world, what are considered the best clean tones are almost always 6L6 based.  

The Prima Luna integrated uses EL34, 6L6 or KT88.  The KT88 is a fantastic sounding tube but I stopped using them due to reliability issues.  They’re harder to find and their manufacture seems inconsistent.  I’ve had several crash on me for no reason other than I installed them.  But, when working, they were the most hi-fi, clean, round bass, beautiful mids of all the tube types.  

EL 34, which is very popular in guitar, is probably my least favorite in hi-fi.   They’re overly warm, with too much emphasis on the midrange.  But other people swear by them. They’re able to handle a lot of wattage and they’re known for their reliability in demanding, high plate voltage environments.  

My advice, if you’re young, is to buy whatever sounds best in your budge but try to get a system, like Prima Luna, that can handle different tube types.   If you get bored, just roll tubes,  it’s like getting a brand new system.  Chasing tone through tube swaps is a life long endeavor.  I have boxes and drawers full of old tubes.  It’s a great hobby.  Enjoy!!! 
 

Many great suggestions mentioned. I might suggest you watch Roger of Rogers High Fidelity YouTube videos on tube amps & what he feels are important. There are a few really good ones . He knows his stuff! Obviously, he’s also promoting his own brand but very worth it.

His offerings are relatively expensive but in his case, you get what you pay for. I have now for a year his EHF 200 Mark II integrated amp & it is outstanding! Very powerful w/great detailed beautiful tone w/ very good extension at both frequency extremes. It does run hot operating in full class A producing a true 100 watts + / channel but that’s the price you pay for that design & the sound quality it offers.  
 

I’ve owned tube amps & preamps for 40 years; Conrad Johnson, Audio Research, Melos, New York Audio Labs, Art Audio & enjoyed them all. The build & sound quality of Rogers handily beats them all. 
 

My thoughts on important aspects of a a tube amp are: 1- high quality, large power oversized transformers, 2- point to point wiring w/ good wire ( much easier to diagnose & repair should a problem occur) , 3- solid, non resonant  physical construction w/ high quality input & speaker connectors. 
 

enjoy the tubes! Warning once you go there, there may be no turning back.