Is it a line only or + phono? If the latter, I definitely preferred the MK3 phono. If only line, I'd think twice about the upgrade -- BUT, if you're prepared to pay $700 (and have the money readily available) you should go for it and think about retubing later.
Finally, if you can't get retubing out of your system just buy Svetlanas or Harmonix for now and worry about NOS later.
Good pre, BTW!