Interesting. I guess if I wasn't so busy at work, I might go for the IRF. I might still, so thank you for that pointer The Sylvania and GE 12AU7 tubes I have tried, are only marginally better than the stock Shuguang tubes. The GE 5-star 5963 was noticeably better. Long black plate tubes, even better than that. Of the old American tubes, I'd have to say the Tung-Sol tubes are probably tops. You need to go to Amperex, Telefunken, or Mullard tubes to improve over those. The quality among those three European makers is tops; the only choice is whether you're looking to brighten something up, or tone it down. I had to distract myself from some family issues over last summer, so I did what some might consider foolish, and started collecting 12AU7 tubes (and its plug-in variants like 5963's and 5814's). And rolling them in my stock Bravo V2 I left some tube rolling notes in this thread, along with another member's impressions. That may help you decide, should you continue that effort.