Thanks for the info. The stock op amps that come in my DAC + headphone amp do not have an actual ground connection, but some of the official upgrade op amps do have an actual ground wire and a place to connect them to earth ground. So I assume this means I should have a real ground I can use. The ground is really close to the op amp socket, so I was planning on crimping a ring terminal onto the end of one of the legs of each capacitor to connect to the ground screw and then solder the other leg of each capacitor to the proper power pin on my op amp adapter PCBs.
I do not recommend crimping terminal rings directly to cap leads, use a short length of wire instead. I use 24awg or 22awg Mogami wire for ground leads, whichever I happen to have on hand. These stranded wires will take more punishment than solid capacitor leads, unless you intend to swap in a different opamp only once and never again.
If the upgrade opamps in question are audio-gd's HDAMs, some people like those best with ungrounded decoupling and a 1uF Soviet K42-Y2 film capacitor across the power pins in place of the original EVOX MKT capacitors. I got to try that out earlier this year, and Sun is even more fun and higher fidelity that way, but I sold off Earth and Moon long ago and can't try those with upgrade caps. The little grey EVOX caps they normally have don't bring out the best sound in Sun at all, I have no doubts that the same applies to Earth and Moon.
As nice as audio-gd opamps are, though, they don't beat TI and ADI chips in sound like OPA827, ADA4627-1B, LME49990, or OPA1612, but they are indeed a nice way to flavor things up a bit. I recommend Vishay MKP1837 decoupling caps if you go with any of those chips I mentioned, best film caps. Or WIMA MKP4 if you can't source those.
Edit: MKP1839 is fine, too, just as good as 1837.