BAT REX 3 with Harbeth 40 series

I take it you don't have the opportunity to try the combination.  I've heard the 40.2 at a local dealership that sells only tube amplification.  This is an extremely touchy speaker that can sound soggy and lacking in dynamics and loose in the bass region with some amps, but not others, and it not necessarily the case that the more powerful amp sounded better with this speaker.  This speaker, more than most, requires auditioning with the particular amp in mind (I don't know if this is the case with solid state amps).

I don’t have a local dealer unfortunately.   I have tried a 250 watt tube amp and it’s quite amazing with the Harbeths.  

i have found the mon 40.2 needs solid state amplification to do the bass really right - have not tried victor's bat's though... they should have among the best damping factor that tube amps can provide, along with my arc amps, but best tube amp damping factor does not remotely equate to that of top tier ss amps

I heard it sound very good with certain tube amps, so I don't think it is necessarily a solid state amp only speaker. I really don't know of ANY speaker that I would call a solid state amp only speaker.  As to damping factor, this measure (8 divided by the output impedance of the amp), doesn't really matter much beyond a certain number.  I have heard amp designers say that number can be as small as 8 (output impedance of 1 ohm), and I've heard amps with substantially LOWER damping factor than that sound terrific with some supposedly difficult speakers. 

This makes it quite difficult to realistically predict how an amp and speaker combination will sound--there are no good objective guidelines. 

I have not heard the REX, much less heard it with the 40.2s, so I cannot so much as guess how that will work out.  But, I can say that an amp should not be automatically ruled out because it is a tube amp, or because it doesn't have an output in the multi-hundred watt rage, etc.  

Overall, even with the better tube amps that I heard with the 40.2s, it is not the most dynamic and lively sounding speaker--it does weight, large scale and harmonic richness very well--but it is a bit of a sluggish thing.  If I were to take a guess at to what might work best with it, I would think that an OTL amp might do the trick--something like an Atma-Sphere amp.

A few months ago I hooked up my new bat Rex three stereo power amp to my  harbeth 40.2 anniversary edition speakers. I was astounded at how much better the harbeths sounded with the Rex 3 when compared to previous power amplifiers that I have used including Pass, Macintosh and PS audio (BHK stereo). The sound was so much better in ALL of the golden ear categories – clarity, musicality, punch, soundstaging. etc., It wasn’t just a small incremental change that I have been used to when getting an improved power amp - it was a quantum leap that I did not even remotely expect and  was by far the most improvement that I have experienced with replacing a component in my 50 years of being a high end audiophile.

Hope this answers your question

I don't have the Harbeths but my REX 3 sounds wonderful with my Fyne 704's. 

My 704's have upgraded binding posts, Internal wiring and crossover components.

The 704's are really nice stock but these upgrades took them to a new level. DIY SP

I also tried the REX 3 briefly on a pair of Martian Logan Montis.

No issues driving the ESL's as far as I could tell. 

I’ve owned the Harbeth M40.1 speakers for the past 12 years and I recommend driving them with a powerful solid state amp with a tube preamp.  I’m using a McIntosh MC452 (450 w/Ch) with an Aric Audio Motherlode XL balanced tube preamp.  In my opinion, the REX 3’s 80w/Ch isn’t enough for large scale music at high volume with the relatively inefficient M40s.  By combining a powerful SS amp with a high quality tube preamp you get the best of both worlds.  My preamp uses 6SN7 tubes in the preamp stage and 5U4GT rectifier tubes that give me a lot of options to tube roll.

Here is a discussion of the M40.2. https://www.whatsbestforum.com/threads/harbeth-monitor-40-2.25222/

The author uses 2 Benchmark ABH2 in bridged mode that puts out 400w/ch.

Here’s another discussion about the power handling of the M40, unfortunately the video is no longer available.

https://hifihaven.org/index.php?threads/how-much-amp-power-do-i-really-need-video-alan-shaw-of-harbeth-nsf-set-fans.1276/

The CH Precision power meters in the video were showing an output of 700+ w/Ch.  Alan Shaw was shocked by how much power was going to the M40s.    I have a large room but I don’t play my music nearly as loud as the video showed when I watched it several years ago.  The power meters on my MC452 have shown that the M40.1 speakers draw a lot of power.  I think you could get by with an 80 w/Ch amp in a small room and not too loud volume levels.  Even so, I would get 2-3 times the power that I think I would need to provide a lot of head room so that the amp is at ease at loud volume levels.   You can see my living room system in the virtual systems.   My 2 cents.