I would say if amps as big as the MB-450s are not delivery the goods, you need to look at other tube amps. I've heard a few hybrids on the VSMs, and then generally do not do what a amp like the 4BSST does on the VSMs in the bottom end.
The amps I have heard on the VSMs that can do that kind of very authoritative bottem end are big OTLS and a few high power tube amps (the Joule VZN-160s and the big CAT amps). Now I realize that most folks would be happy with the bottom end a VZN-80 or the VZN-100s would deliver on the VSM-Ms, but the bigger amps have more authority, weight, and defintion in the lower registers along with the great capabilities these amps offer in the midrange and top end.
FYI, I have the latest retail price list from Joule-Electra.... a pair of VZN-100s is now $15,000 ($18,000 in musicwood). Used VZN-100s are hard to come by.... Joule's production capabilities are limited, so there are only so many out there and the customers I know who have them are very attached to them. :-)
Jim2, you are right about the VAMP.... given the way Jud builds things (allot of labor goes into the fabarication of each Joule amp), he was loosing money on the VAMP.
Happy listening,
Rich Brkich