I had a VooduValve for a bit - I didn't like it at all. I got it for it's speaker simulation and that was poop. Also, the control layout made me batty. Completely counter-intuitive Haven't heard the Prophecy so no opinion there...
It's got a single valve, but the actual distortion comes from clipping diodes, not the preamp tube. I think the tube is used to dull the highs slightly.
The trick w/ the voodu valve is to turn on the speaker simulator run it through a Tube power amp, and then into a 4x12 cab. In that setup it positively smokes. Anything else and it's going to sound like poop.