I picked up a Bogner Shiva and I went through the following: - I fired the amp up and all was well for about 5-10 minutes of playing then a very noticeable hum/hiss began. This isn't my first tube amp by any means, so I know there has to be at least something wrong. The sound was very shrill and steady. It was also on both channels, therefore I believed it to be a microphonic tube in either V1 or the PI. - I tapped on every tube with a pencil eraser while the amp was on, however I couldn't really hear a noticeable tapping sound coming from the speakers. - I let the amp cool overnight and took the tubes out today. One of the 12AX7s (an ARS Chinese) has a very noticeable jingling when it is gently moved around. I believe that this tube is my huckleberry. - I moved around all the tubes and put the believed culprit in V4 to see if there was a difference. I started up the amp and played through it really quick and the sound is completely muffled; the channel and master volumes needed to be very high to even get a level lower than what could be considered bedroom volume. This to me confirms the idea of a shot preamp tube or tubes. - Summary All tubes light up There was a very shrill and steady hiss/hum I checked the tubes and one 12AX7 had a noticeable rattling I moved the tubes around and the amp is now incredibly muffled (makes me believe this is a preamp problem) I am going to order some new tubes from Doug's Tubes, but prior to doing so, I wanted to see if anyone had any other ideas about what could be going wrong with the amp.