I have been playing around with a whole range of driver tubes on my WAvac EC300B. There are audible differences that are of the same order of magnitude as output tubes. Specifically Jan Philips 7581a: very resolving and fast, yet slightly harsh in the treble. Svetlana Winged-C 6L6GC: very harmonious, sweeter than Jan Philips 7581a, yet most depth of soundstage, Tungsol KT66 (actually really 6L6 GC with 900mV heater current): deepest bass, very powerful but slightly sluggish. These were the best of the lot; JJ, GE and RCA black plate are fuzzier and less dynamic. I have not yet tried the Tungsol 6L6GC reissue but am intrigued because of the strong KT66 performance. As mentioned mixing and matching driver and power tubes gives additional mileage.