Most “disappeared” speakers.

very true about ESL's.  one way around dynamic point sourcing is by using multiple active speakers.  4 beolab 3's with a beolab 2 work extremely well in this regard in one of my rooms.  not so easy or achievable with passive speakers, unless they are hybrids with some ESL component

Electrostats combined with a good sub - sublime. 

OB’s, aka Open Baffles, aka speakers w/o a box design. Combined with 2 subs and a low watt tube amp, very tough to beat for quality sound on a reasonable budget. 

Lx521. Sound field is wide and realistic 

In that price range new and used you have a lot of choices in vanishing speakers. I was lucky enough to find a pristine pair of Maggie 3.7, sent them back to Magnepan for a total refresh and they emerged as the lastest model the 3.7i all for less than $4K.
Just be aware that the more transparent a speaker is the more it will reveal the shortfalls in your sources and supporting equipment. 

The omega super alnico without any crossover is a nice choice for a disappearing speaker. 

With a sub and low watt single ended amp it is sublime.