And what could this "mod" be? Maybe one of those small, unannounced changes / upgrades Mr. Moffat has recently admitted they are making from time to time?!
Now the big question is this: does your Gumby sound any different now, apart from the lack of static and popping sound? (I mean in terms of detail, tonality etc.) I'm asking because I've had some matching problems with the rest of my gear in terms of timbre and tonality and I'm wondering if this problem had anything to do with it.
As for the Dc offset measurement, the procedure goes like this:
1. First you need a multimeter. Even a cheap one will do.
2. Play any piece of music through your Gumby. It doesn't matter what, but it's essential that the outputs are "live" when you conduct the measurement, not shorted or disconnected or whatever those relays are doing when there's no signal entering the dac. I think (I'm not trained in electronics, just an amateur) that it's better if the dac has been warmed up (for a few minutes / tens of minutes, not necessarily days).
3. Turn the multimeter on and set it to measure DC voltage, for instance set it for up to 20 V at first and then you can use other, lower scales. In
this picture, for example, the DC voltage scales are in the higher left quadrant, below the "hold" button.
4. Insert one of the multimeter lead tips into the center hole (signal) of one channel's RCA output connector, making sure you touch the metal inside, and touch the other lead tip to the outer jacket (ground) of the same RCA connector. Write down the value shown by the multimeter and then repeat the procedure for the other channel. Alternatively, it might be easier for you if you conduct the measurement at the free end of an interconnect cable
connected to the RCA outputs. As long as the cable is functioning properly, it doesn't influence the reading.
Apologies if I'm too much into stating the obvious in the above paragraph, I mean no offense - I just have no idea what is your level of knowledge.