yes it is microphonic, switching the volume controls up and down sends the clicking of the stepped attenuators right thru the system, but it ain't no big deal really.
The sound is worth it, and yes you do have to find the right tubes.
karl has a way to test for microphonics and to match accuratley that uses a computer, he says the Hickocks are a joke for real testing of tubes. They just do the basics, when dealing with DHT tubes a bit more is needed. And yes his Cunningham 26's sound great as well.
Fortunatley these tubes are pretty cheap at 5 and 10 bucks a pop. Buy a bunch and test them to get a matched pair. In the world of $300+ Tungsol roundplates, buying a bunch seems a bargain
mike