I got a Cl-38 uc recently. Opened it up and saw that the preamp stage tubes were basic JJ 12au7's - 5 of them arranged in this fashion:

: : .

2 tubes - 2 tubes - 1 tube. I have a pair of NOS Mullards, which position would be the best for these, or do I have to replace all 5 for any benefit?
Cheers,
Sid