Best Tube Amp for High Sensitivity Speakers: 45, PX25, or 300B?

I think the OP is mistakenly ascribing personality to the 45, 2A3 and 300B valves. What matters more is the circuit design and parts used. I have a 45 SET custom-built by Will Vincent (autospec) on an ST70 chassis that turned out beautiful! Plus a Japanese Sanei SA-560A 2A3 SET made with Tango iron. This I won at an eBay auction from a Japanese seller. It is the quality of the whole amplifier - the gestalt - that elevates the performance rather than the specific tube type!

I like low-powered tube amps of all designs--pushpull, SET and OTL--so I am not so wedded to a particular design.  As jasonbourne71 stated, it is the particular design and parts used that matter most.  In particular, the output transformers and interstate transformers (if used) that matter a lot.  

I do have some tube preferences (meaning I've heard a number of good amps made with a particular tube type), but it is not absolute such that I universally prefer something made with that type of tube over others. 

Of the three small triodes mentioned, I tend to most like the 45 as long as it is used with suitable speakers.  I own a parallel 2a3 amp that I used with 99 db/w efficient speakers and a 45 pushpull amp (both have sufficient power for my purposes, but then again, I don't play the system very loudly.  I currently run a pushpull 349 amp (5.5 watts per channel) that I like a lot.  I tend to prefer the 45, 2a3, 300b over the larger transmitting-tube SET amps (211, 845) although these can sound very good.  For pushpull amps, I also like a number of old and new designs that employ 6L6 and KT66 pentodes.  For really exotic tube types, I like the 350B, 349 (pentodes) and the 252 (triode somewhat like a 300b).

For the OP's 96-97 db/w speakers, it might be possible to use the three triodes mentioned (particularly the 300b), as long as the owner is willing to accept some restriction on very high volume listening.  I personally feel that some compromise in that area is worth it.  For a safer bet on compatibility, something in the 40 watt range should be sufficient.  

Look for a Dennis Had Inspire 300b. He builds each by hand one at a time, essentially made to order.  Sells on ebay under radioman731.  He built me a KT88 last summer, added a volume pot so I can stream directly to the amp, then straight to Klipsch Forte 99dB efficiency or Focal Aria at 92dB.  The KT88 can run 6L6, 5881, EL34, KT77, 6550 and KT88 @8-10 W depending on output tube. Pure heaven with the Klipsch. His 300b comes fit with the new production WE 300b for $4,250 and is rated @6W per channel. I was able to pickup in Cary, NC and he treated me to lunch with an audio buddy, tour of his shop the Toyfactory, plus an afternoon of music and career stories.  BTW, he founded Cary Audio, credentials enough! 

One of my 'under-powered' friends has this Art Audio PX25, 6 wpc

https://www.enjoythemusic.com/magazine/equipment/0801/artaudiopx25.htm

Noooooooooooooo, Nooooooooo, Nooooooooo

I currently own a Tektron 2A3/45, MastersounD 300B, Art Audio 845 and most recently just sold an Allnic m311 (PX25). I utilize beautiful Coherent Audio 18's that have a similar level of sensitivity @ 98+dB. My listening room is a bit larger, but overall, this question is one that I have experienced directly for many years. But, before I actually post, I want to share that I have been reading blogs for many years, but have never actually commented as I find the audio journey such a unique and personal experience. All I can profess to do is share my own unique and individual direct experiences. 

I love my Tektron 2A3/45. These low powered tubes built by Attilio in Italy are a wonderfully designed and built, very reasonably priced gem. The tubes (2A3 and 45) are to me the epidemy of SET sound. They deliver an open, airy, detailed experience that is intensely close to an intimate live performance. However, and for the record, there always seems to be a however, in some genres of music, they can just lack a complete and full body. For an intimate setting of both female or male vocalists, or even the delicate concert delivered by a cello, I find nothing even compares to these tubes. But, when I play a full orchestral rendition of a moving concerto, or a little rock and roll, they tend to fall slightly short in the arena of complete body and tonality throughout the frequency range. Please don’t mistake full-body with bass. When in a proper setup, this amp has plenty of fast, full lower frequencies which I have never felt lacked bass.

My MastersounD 300B however, is fast, dynamic and never has lacked for power, regardless of the demands placed on it from the style, genre or volume of music. It sounds more truly complete throughout the frequency ranges, but and here we go again, lacks that true SET quality. I know I might be committing audio Hare Kari, but to me, it does not have the detail or the delicacy of the 2A3/45 tubed amps.

The Art Audio (845) maintains more SET true nature than the 300B, albeit, less than the 2A3/45. It actually has a sound closer to my Tektron amp than the MastersounD does. It delivers amazing power with full body and a force that is not touched by either of its rivals. One of the many beauties of the Art Audio Carissa is that it does not overdrive the tubes and therefore does not get any hotter than my 300B tube gets. I know, once again, the comments seem hard to imagine, but when you speak with Art Audio you will understand their reasoning and why the amps do not get hot like most all other 845 amps. But, to me the 845 just lacks a little tonality throughout the dynamic range. Perhaps just a tad rolled off in the highs, especially compared to either the 300B or the 2A3/45 amps. It also loses just a touch of detail and delicacy found in the Tektron; however, it delivers amazing full body force and punch that is spine-tingling during a moving classical overture.

The Allnic m311 (PX25), so sorry, I actually have never heard the Audion PX25, but owned the Allnic for man years. Was a great amp. It delivered more punch, power and body than the 2A3/45, more SET like detail and delicacy than either the 300B or the 845. However, to me just lacked a little excitement. It always sounded good, very good, but rarely if ever stopped me in my tracks and caused that hold your breath pause that comes with music being played through a piece of art.  Still, the Allnic is perhaps one of the finest amps I have ever had the pleasure of owning and a true joy to listen to as a daily driver. However, getting PX25 tubes can be a challenge. KR Audio is the only current manufacturer of these unique tubes and they only produce tubes when backorders are substantial enough to warrant the process.

This week I will be taking delivery on an Art Audio Jota, a high-current set of monos that delivers 20+ watts through a unique 1605 power tube. I am hoping that they deliver the “unicorn” I am seeking; SET like sound with a bit more punch. Until then, I am a spoiled audio aficionado that switches between my 2A3/45 and 845 as the music and my mood demands. What a great hobby we all have!