Not sure what the gain of the Koetsu is, but my guess is low.
So, first of all, stick with the 12AX7, 12AU7 tube complement.
I would recommend starting at 100 ohms loading or even 50 and listen. You can go up from their. Quite often, lower loading values will sound best with GOOD MC's, such as this, but let your ears be your guide.
Thanks,
Dan