2a3 ?

I like 2a3’s, provided the speaker is efficient enough.  The. 300b tends to deliver a richer sounding midrange and a warm bump in the upper bass that is part of the “magic” of that tube, but, compared to the 2a3, the 300b sounds murky and not as dynamic and alive.
At even lower power, there is the 45, which sort of fits between the two.  It is not as lean sounding as the 2a3, it delivers deep bass (but tighter and punchier than the 300b), good dynamics, and almost as much clarity as the 2a3.

When my local Audio Note desler first set up the ultra expensive AN-E SEC, he chose Audio Note’s Kageki (parallel SET 2a3 amp).  He used the same amp when setting up the top AN-J model.  The sound of both speakers was magnificent.

i own the Kageki and like it, although these days I run my pushpull 349 amp.

My speakers are 99 db/w efficient.  I own a pushpull 45 amplifier that works well with these speakers, but, I don’t play music that loud so I can’t say how happy others would be with my setup.  I have a parallel single ended 2a3 amp that also delivers sufficient power.  My favorite amp is a pushpull 349 (this is a very rare tube that, fortunately, has a long life when driven gently) amp that delivers about 5.5 wpc.  

There are fans of the romantic sounding midrange of the 300b, who will not listen to anything else, while there are people like me, who like the tube, but not as much.  You simply must listen to a variety of amps using the various tube types to make up your own mind.  

All of these simple triodes are most often employed in single-ended amps, but they work well in pushpull; the sound is different for the two topologies.  Pushpull tends to have tighter, punchier bass and a better sense of drive, but, single ended amps tend to sound more natural, relaxed and less "mechanical" sounding (the bass punch of pushpull can sound the same all of the time, hence mechanical, while single ended amp bass seems more subtly variable).  I like both types of amps.  As I mentioned above my favorite is a very low-powered pushpull amp, but my all time favorite amps (that I don't own) include an output transformerless amp and an extremely vintage pushpull amp that runs a 252 tube (variant of a 300b).

I have heard some SET amps running the big transmitter tubes, including a 1610 tube, and I have not heard any that surpassed the lower powered SETs when playing speakers of high enough efficiency such that no amp was being forced to play out of their comfort zone.  Yes, some 211, 845, 833 amps sound pretty good, but I still like the 45, 2a3, 300b amps a bit more.  

I never ran my parallel SET 2a3 amp into hard clipping, but, I did suspect it was slightly compressing peak volume when playing certain kinds of music.  This appeared when playing large acapella choral works at somewhat high volume--no obvious distortion but the music sort of stopped getting louder at peaks (99 db/w speakers).

I don't agree that, even with very efficient speakers, SET amps always win over pushpull.  There are certain pushpull amps that deliver the same sort of natural, harmonically dense and complete sound of good SETs, while sounding a bit more punchy and dynamic.  There are also contenders from a completely different camp--the output transformerless (OTL) amplifier.  The incredible dynamics and liveliness of good OTL amps can be extremely captivating, and they deliver this kind of sound without necessarily being bright or harsh.  I don't think any particular topology beats another; it is a matter of taste/priorities, system matching, and specific execution of the topology.