Harbeth vs Vandersteen vs Sonner

Yea, who decided to let amputees into the Olympics? Why should they be allowed to compete? They should know their place, right?

Wrong again.

Joke
Your head

To the OP , great question- take a look at my Casa Pacifica  system in the virtual system section for an idea of room size. I don’t listen very loudly to the Treo CT but every now and then do crank them up into to mid 90’s. They are engineered to be an easy load and can be well driven by a quality 40 wpc class a/b amplifier. I use a NAIM atom but also have a 100
wpc Music Reference RM-9. It’s not a loudness thing, but it is an amazing amplifier- created by a genius ( RIP Roger ) I think there are many many quality SS and Tube amps well suited to Treo. My short list would include: arc, Music Ref, Rogue, VTL, Aesthetix, Ayre, Belles, Line Magnetic... NAIM, I only recommend things I have heard
fire away w questions 

100W is fine for all three.

Harbeth is a highly renowned manufacturer of well regarded audiophile monitor like speakers in the BBC tradition for 40 years. They are currently selling at a premium all they can make and orders were 8-12 weeks out before the pandemic hit.
Vandersteen is a bootstraps company that is in the conversation for top 5 of the best, most innovative full range speaker systems and is a household name among audiophiles.
Sonner Audio appears to have been making speakers for 25 years. The Legato Duo is an attractive speaker with an aluminum woofer, paper cone midrange and ring radiator tweeter. I would like to hear them in an extended listen with a wide variety of recordings. On paper (no pun intended) I would be concerned how the handoff between an aluminum woofer and paper midrange would work, especially at such a low crossover frequency of 200 Hz. Aluminum woofers are about as slow responding as you can get and paper drivers are about as fast as you can get. The concern would be for proper blending and coherence through the crossover frequency regions.
I have heard many ring radiator tweeters and while they can sound very good, they are often loaded with too much sizzle that can make for a distorted sound if not properly managed in the crossover. They also tend to be less omni directional than dome tweeters which could lead to a more narrow sweet spot both horizontally and vertically.
They may sound great at a demo but an extended session would be needed before I would choose them over Harbeth or Vandersteen.
FYI my pick would be for the Harbeth SHL5+ (non anniversary, which are in the range of the 30.2s).
I have the privilege of owning them paired with Rogue Audio separates
and the sound vs. the 30.2s is more open, transparent and with deeper bass response.

Pick one and buy it ASAP!!