I havn't heard the amp. However I have followed the story of the amp over at DiyAudio.
Nelson started using Power Fets from a company called SemiSouth. These are different critters
than the typical MosFets, they are more of a power version of the Fets he uses in his pre amps.
He worked with SemiSouth and commissioned a Type of PowerFet called a Static Induction Transistor,
Sony made some a few years ago but these have a higher power handling capability.
The transfer curve on these babies looks like a DHT, think 300B or 45.
He is making a couple of low power (10W/8ohn) First Watt amps.
Had to cost at least a few hundred thousand dollars to have a semiconductor line set
up to produce these Fet's. I've never heard of an audio company having Semiconductors
designed for their product.
Link to Nelson's description of the device:
http://firstwatt.com/sitintro.html
Love to hear them. Could be special.