The splashy high freq are likely caused by no bypass caps on the S-200 not the 5.5. I went through the same symptoms with a TL6.5 II and the S-200. Swapped amps and preamps to determine the cause.
Added a couple of .1uf film bypasses to the S-200 filter caps and most of that splashiness was gone.
As for 5.5 preamp tubes, you may want to consider the RCA 5814 black plates in place of the 12AU7s, and RCA or Raytheon 6201s in place of the stock 12AT7s.

