Etymotic are very neutral, and I don't like the deep fit. I don't think it would be a good match with a306, which is already neutral- neutral bright. There's so many chi fi iems too. That fiio looks nice, and I wouldn't mind trying the Sony F7 or F9 myself.
Anyways just to mess around I've got a 4 pole 3.5 balanced to 2.5 balanced adapter coming today. I don't expect anything but maybe a little cleaner output and maybe a bit better crosstalk. My curiosity got the best of me since sony said they were using a 4 pole jack. Better grounding, probably nothing audible will come from it. But it was cheap so... BTW I tried my ibasso dc03 pro today and it sound so dark compared to the Sony, and the Sony still has great bass, it's just really punchy and clean.
Pic of adapter- I've used this brand before and they're pretty decent.