After reading up on reviews and this thread, and wanting to try IEMs for the first time after the left speaker on my HD595s went dead, I decided on the Atrios. I received them about two weeks ago and have been listening to them every day since.
My first impression? I couldn't believe how low this bass goes!! I've never heard anything like this before in something so small. It's unbelievable. I've yet to hear any frequency too low for these not to reproduce. They absolutely smash my HD595s in that department, and even beat them in overall clarity (but not in soundstage, as is expected.)
I've had time to burn them in, up to about 40 hours or so as was suggested. I actually didn't notice too much of a difference with burn-in, but I'd say the sound leveled out to be a little more natural overall.
I only get a great seal from the large "Earfills" flanges. I don't find them uncomfortable at all. For me, they give far better sound than the included foams, mainly because I just couldn't get a seal with those.
I haven't heard the SA6s yet, but I am absolutely satisfied with the Atrios. They sound spectacular overall. It's just awesome to be able to watch movies and get that rumbling bass that you'd only expect to get with a dedicated sub. But it wasn't just the bass that blew me away. I expected I wouldn't get very good mids and highs from phones that were so low-end capable. But they shine on the higher frequencies to boot. They don't quite have the separation and sparkle on the highs that the 595s had, but considering the mind-numbingly low bass, I think they do more than well enough on the highs. They've got a very smooth mid-range to them that I've found to be easy on my hearing for long periods of listening. But the fact that I can even compare them to full-size headphones at all should definitely be an indication that these are some bad-ass IEMs!