Harbeth 40.1 vs Sonus Faber Olympia III

I agree with djfst that the 40.1’s don’t need a lot of power. I’ve driven mine with as little as 32 wpc tube amp (VAC 30/30). You ARC Ref 75 SE will be fine.  I'm currently using a Music Reference RM9 MK2 EL34 based tube amp rated at 125 wpc.  

I heard both speaker there is no competition at all the Harbeth is much better than the Sonus .

There's a like new pair of 40.1's listed for only $6,900.   That's a steal.   You'd be hard pressed to find a better full range speaker for that price.

Harbeths all the way. Wonderful speakers.

A speaker preference is personal. Having heard both, I’d personally go with the SF.  The Harbeth is not “a much better speaker” -  it is a different speaker and they both have their strengths.   FWIW, I own neither right now but the SF O3 is on my list, along with the Serafina’s,  KEF Ref 5s, and Vienna Acoustics.  I’ve got some room challenges that I’m taking into consideration. I’ve listened to Harbeth a number of times compared to other speakers and while yes, they have a nice midrange and pretty sweet treble, they weren’t the most satisfying to me at their price range.  I will also say this - I have to live with and look at a speaker as part of my living space as I don’t have a dedicated room.  In that regard, the Harbeth’s are at a very serious disadvantage to SF.