BAT REX 3 with Harbeth 40 series

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.