EVO and XE6 are very different IEMs. I would agree with your characterization of how they differ.More extension at both end of the spectrum?! I can imagine XE6 having more treble extension but can’t imagine anything reaching deeper than EVOs bass… from what I’ve read the XE6 goes for more of a mid bass emphasis and lacks a little extension in the sub bass compared to other TOTL in this area. That’s the impressions I got from the posts I’ve read anyway.
You can absolutely go wrong buying either of these IEMs. It may not be your flavor. I loved EVO out of the box at first listen. It took me a week of listening to like XE6. That's mainly due to my preference for sub bass and more separation between bass and mids.