You can test DC offset without any music, which makes aeberbach's point moot.
I suppose it's possible that in some circuits that the DC offset could shift when there's an AC signal present, and so to be safe I guess you should test that, too. To avoid what aeberbach is talking about, use a pure sine wave as your test signal. That way the AC swing in each direction is the same. I haven't tested this, but it seems to me that as long as the period of the wave is higher than a few Hz, it shouldn't show up on a DC meter.
Also, consider retesting after you jumper C1. This time, test for DC offset on the output of the amp when plugged into various sources. If the source has very little DC offset but the amp amplifies it (that's it's job, after all) to significant levels, you still have a problem. Also, the META42 will have its own DC offset, which can be significant depending on the amp's resistor configuration and the op-amp you're using. You need to test all of this in combination before you plug headphones into the result.