Retrovalves

Not thinking of using these, but does anyone know what they are and how they would work.

http://www.robertsaudiotech.com/designs/retrovalve.php

They would seem to be high voltage solid state devices made to fit into an existing valve base.  Probably MOSFET I would guess.  At first listen they seem to work fine.  Are they exactly the same sound as a normal 12AX7 valve?  I'm not one to pontificate on the differences in tone between a left handed and a right handed resistor of the same type, (sarcastic joke , most differences of that type are non-existent and even those which do exist are so minute that, to me, they pale out of any significance in a live situation with a band and that's all I'm interested in.)

The interesting thing is not that they are doing this, MOSFETs have been used alongside valves for a long time, it's that they have manufactured a direct replacement in existing circuitry.  And of course there is the ability to actually choose parameters which there is not in true valves which are all pretty much the same within a small manufacturing tolerance.  (Just look at the spec sheets and the response graph comparing the many makes!  Oh no, wait a minute, that would be an engineering thing which wouldn't allow for the much beloved mojo. )