I changed to amperex 7308 white label, and tungsol 5881, I spent $600, but the result was fenomenal, a impressive change in qaulity of sound, sweet and a full bass., some time I tried a inexpensive jan phlips 6922, but the result was poor. You can see that almost all bat vk5i woners recomend amperex and tungsol, these tubes make another kind of preamlifier.
Good luck
Good luck