Who are the most underground high-end companies

Enlightened Audio Design (EAD), Dunlavy, SVS just to mention a few.

If I hear about it (and especially if I own it), it is no longer under the radar. Seems more common in Europe or Japan. However, if I can suggest a three names that have not been mentioned

Burgess
Border Patrol
Ancient Audio

The first two that come to mind are Convergent Audio Technology and Spectral - very little advertising, but generally recognized as making fantastic products.  If you want to go further off the grid, Wavestream Kinetics (product of Scott Frankland, the "F" from MFA) makes fantastic preamps, but again, almost no ads or publicity.

Quicksilver Audio,doesn’t advertise or loan for reviews. Outstanding gear!
Croft Acoustics is another one!

Exemplar Audio