I'm not sure, but this may be of help. The clicking noise is probably the output relays. 1. You mentioned volume dropping about 50%. Could it be one channel dropping out? 2. When you turn on the dac, how long does it takes before music comes out? By design, there is a delay circuit that allows the tubes to warm up before playing music. When everything is perfect, there will be a delay of some seconds (30 sec?) followed by a click (relays) and music. When I got mine, there was no delay and in a few weeks, I started to get full drop out and eventually no sound at all from the SFD-1 Mk2. Not knowing anything about the circuit, I sent mine to Canada and they changed the electrolytic capacitors in the timing circuit and everything is fine. Those capacitors are C501 (33 uF, 16V, 85 C) and C502 (47 uF, 16V, 85 C). The capacitor names are marked on the circuit board and are located at the back of the circuit board, below and slightly left of the input circuit board. The capacitors are not in the signal path, so audio quality isn't important. I do recommend replacing them with the same value caps. PCX put 100 uF caps in mine because its what they had on hand and the delay is about 90 seconds. If you have no or low delay for music when you turn it on, then the caps below are probably the issue. That wouldn't be surprising given they are small electrolytics that are nearly 30 years old. Actually, all of the electrolytic caps in the unit are candidates for replacement and Ed's excellent documentation of his DIY version of an SE upgrade also accomplished replacement of all suspect caps. If your problem is only in one channel, then it still could be the caps with one relay opening at a lower voltage than the other. Also the relay itself could be the problem. There could also be other problems in that circuit from the chip that the caps are close to, to resistors themselves, but my best guess is that delay circuit. Of course, it is a complex circuit, but the clicks suggest the relay circuit. Good luck and let us know what you find.