I just noticed this thread after PM'ing you and your lawyer about the topic of the best tubes for preamp usage. I have found that the 2c51 driver and 6BL7GT output is really good and still allows for a reasonable volume range. I previously found that all 6SN7s with my old MPX3 was a bit soft and unfocused, but with 6414 driver and 6BL7GT outputs that one sounded good but the gain was high. If he runs into trouble with the 2c51, he can always try a K-R black glass or Tung Sol round plate 6SN7 as the driver and see if that calms it down for quieter listening.
As I said elsewhere, he was genius to ask for your services rather than sending the amp to SP for the pre-outs. I am not a DIY guy, but I really don't see why you couldn't wire them just as you describe.