Im not sure which high end ESC your talking about that would allow the user to set the BEC voltage to precisely 6.6V, but just quickly going over the most popular high end ESC out there...

Orca R32 - 6V/7V
Hobbywing XR10 Pro - 6V/7.4V
Reedy Blackbox 510 - 6V/7.4V
Tekin RSX - 6V/7.4V
LRP Flow X - 6V
Scorpion Vanguard S3-140 - 6V
Muchmore Fleta Pro - 6V
Team Orion R10.1 - 6V/7.4V

None of them can set 6.6v