Most “disappeared” speakers.

The most "full range" dynamic speaker that do the disappearing act would have to be Audio Physics, and then also most ESL’s and Planers.

Cheers George

Easy. MBL speakers.
I got my 121s for a steal years ago, but that was because they were damaged in shipping to someone else so they were sold cheap.
Nothing disappears/soundstages like the MBLs that I've heard.
Then again, it's rare to find them that cheap. 

Otherwise, I agree with george, Audio Physic speakers are reliable for a disappearing act (one of my other favorite brands). 

It's as much about the room and speaker placement than the speakers - maybe more.

+1 Georgehifi

You can try out Magnepan LRS with generous 60 days return policy.

Planar speakers are known for disappearing act.

I also pulled the trigger for LRS although I have nice dynamic speakers.