I posted at various times about tubes in my former 5i. You can do a search and find that info. Basically, I used a few combo's in the 6 input 6922 section ranging from all Siemens 6922. then 7308's. and then Amperex PQ 7308's white label.I also tried all Red Print HM Tungsrams..which were the most transparent of all. In the 2 AC shunt Regulation tubes sections( the 2 outboard 6922's) I used anything from Amperex, Siemens, Tungsrams and stock Sovteks...Not much difference with those tubes at that position to my ears. The biggest difference was the 6 inputs and the 2- 5881's. I wholeheartedly recommend Tung Sol 6L6/5881 here.Very warm and musical.
If I may make a suggestion however....None of the 6922's were stellar in this pre amp. i could have easily lived with just the 5881 upgrade alone, and a power cord upgrade. My suggestion would be to leave the expensive NOS tubes alone here and get yourself 4 matched pair of Russian 623n-eb SLNM Cryo's tubes from ATSI in Florida. These tubes are killer in everything I tried a 6922 in...and displaced even my venerable early type Siemens CCa's in my phono stage. They are quiet,focused and have excellent sweet highs, transparency, soundstage depth, and superb bass. You wont believe they are Russian tubes. The best part is that they sell for $75.00 a matched pair, and although not cheap for 4 pairs...they sell for much less than your typical Siemens or Amperex NOS gems. And the 6h23's will be new!...I cant say enough about them. If I had my 5i now..I would definately have given these a try.