The Vision Hybrid DAC used a version of our Ultra II tube hybrid circuits, with a 6N1P (later a 6CG7) triode followed by a power mos-fet inside the overall feedback loop. This was an excellent and musical unit.
This was replaced by the current production Fet Valve Hybrid DAC which uses a 6CG7 Broskie totem-pole tube section followed by a mos-fet follower, with no overall loop feedback. We find the Fet Valve design to be slightly superior overall for the DAC requirements and have used this for the past few years.
I would not spend the money to replace one with the other, the differences are minor.
Frank Van Alstine