I understand Murray is using a Behringer for crossover and equalisation duties, and for some signal treatment. He has developed an algorithm to pre-condition the signal in such a way that the distortion his valve amps generate are cancelled. Feed-forward transfer function correction I believe he calls it. (Only joking, Murray.)
EDIT: Damn, Murray answered while I was follwing my whimsy.
Marchand do a valve crossover, I believe. Haven't seen one let alone listened to one so can't comment. Generally, if I was in your shoes, I'd try to find a single power amp I liked before thinking about multiple ones. Passive crossovers can go a long way, theses days, and your speakers are expensive enough that Ralph shouldn't have skimped on the crossover components.