I have tried 2 Dario's, 2 Amperex's and the stock Svet in my RT66, and all are microphonic when tapped. Is it possible that something with the amp would make them seem microphonic? I thought the first Dario I got was not microphonic at first, but after trying the 2nd Dario last night, I wonder if I might have had the volume down or the attenuator on when I tapped the first time. I have tried several different tubes in the PI also, but it effects nothing. The PI seems microphonic, until I pull the V1, and then it is dead quiet when tapped. Is it just normal for EF86 tube to be slightly microphonic, and I should not worry about it? I have thought about a new amp, but all the other amps hat I want are EF86 pre-amps also ( Z28, 6545, and Clubman). Plus I love the tone of the RT66. BTW, the Dario's sound much better and warmer than the other tubes. And Mike at KCA has been very helpful and sent the 2nd Dario for me to try. I just want to get some EF86 opinions before I bother him again.