looking to match a new speaker to ARC REF75se

Stereophile measured the SF’s back in the day. 91dB sensitivity, but a pretty demanding impedance that would average 4 ohms.

Not familiar with the ARC so I'm not going to opine.  Perhaps others will.

The upgraded III or Vandy Treo

I have run my 75 SE with Matrix 802 s3 and Matrix 805.  Plenty of power and sounds great.  I also have tried Dynaudio Special 25 with the ARC and EAR 509.  So the ARC has gobs of power to run these.  The amount and quality of the bass is impressive. Midrange and highs make for a very enjoyable listening session.  It is a very versatile amp, used it with both solid state and tube preamps.  Its a special piece and I am thrilled I was able to get one.

The standard match would be Sonus Faber. I have all Audio Research components and Sonus Faber speakers… my third set of Sonus… upgrading twice. They are a great match if you want highly musical, natural, detailed, good sound staging. 
 

Reach as far up as you can, used will get you further. I am listening to my Amati Traditional at this moment… a Cuban band… incredibly beautiful.

First and foremost, what is your budget?

What kind of music do you listen to?

What kind of sound quality characteristics do you seek?

What kind of sound quality characteristics do you want to avoid?

How big is your room?

How loud do you play?

I've had a Ref 75SE for 2 weeks--great amp!  It's got drive and is well balanced so I think it could work with a lot of different speakers.  Depends what you're going for.  I am currently using it with Totem Tribe Towers in a somewhat compact space.  Like the ARC amp, the speakers are detailed but even keeled.  Fed by an MSB Analog DAC, I'm quite happy.