Also, what is complimentary to one, may be unbearable to another. :) Just sayin'.

I think, since this is such a subjective thing, and your client is the one whose opinion matters most, a question I would ask is what was he using to power these speakers prior? Was he happy? If yes, safe thing would be use something from that same manufacturer. If not, then at least we would have an idea of what to compare to. Otherwise you are going to answers all over the board.

I can tell you that B&W would recommend Classe or Rotel as they were designed to "compliment" each other. Or maybe it is just because they own both companies. ;)