Just came back to post an update re: using the Complys with the QC20.
I got some T500s today and couldn't put them onto the QC20 directly. I had to make a small incision in the core/tubing of the tips. Which is probably not a good idea, because if you cut too far in (or if the incision deepens on its own), it becomes too loose around the QC20's stems and you lose all isolation.
If you've got the QC20 and want to try Comply, I guess there's no real off-the-shelf solution. De-coring a version of Comply/Shure Olive with a smaller core, then stretch that over the stem? That might work, but I haven't attempted that yet, so I can't tell you what would work... probably not the T-100s. Sorry for rambling, heh.
The story has a happy ending for me though, because I finally figured out how deep the QC20s are meant to be inserted - Bose's got a video on YouTube - hint hint, it's the same as many other IEMs, not shallower as I'd mistakenly thought - so I'm finally getting the full isolation out of them.