I don't know if there were in fact only 6 made, but I know they are REALLY RARE... I have only seen one in person. It was at a music store in Memphis in '94 or '95 it was pretty beat up. I think the price tag is... "cough"... extreme. The one I saw was about $1100 at the time and they sat on it for quite a while, a year or more... I'm sure it's worth more than $1100, but I doubt it's worth $40k!
As I recall from what I have read, Pete Townsend wanted the 8x12 cab made, but quickly abandoned the idea because they were too heavy and too hard to move.