Rather than change the tube to another, I would suggest using the suggested tube (12AX7) and changing out the speaker to a more efficient one. This will give you more volume at the same settings on the amp and give you better control over the tone. The preamp tubes you mentioned are NOT interchangeable.
If you need help getting a speaker I may be able to give you some suggestions even though I'm living in OZ now!
cheers,
Mark.