If solid, well defined bass is a focus, the UM3X certainly won't disappoint. It has some of the best bass I've heard from an IEM, flat and extended. Like was said, the UM3X is slightly on the warm side, not recessed mids or highs but just a little quieter than the bass line which gives it a warmer sense.
The IE8 is another great earphone when it comes to bass. The driver has a lot of throw and is highly detailed on the low end. There's a good, natural sense to it. The downside is the midbass hump is a bit dominant and instead of sounding warm, it sounds bassy instead. The rest of the response is well balanced for the mids and treble, but bass is dominant. You get used to it though, and the vast sound stage is a good wow factor. I tend to lean slightly more towards the UM3X though since it's a little more dynamic and edgy. You might call the IE8 a little more organic I guess, and people do tend to favor dynamic drivers over BAs at times.
What's better? I personally say the UM3X. I feel it offers just a little bit more overall.
The Triple.Fi 10 isn't quite as capable on the bottom end as the UM3X or IE8. The treble is more emphasized though than either the UM3X or IE8. It rolls off just a tad early at the bottom end and loses some of that deep, visceral sense that the UM3X and IE8 offer. The bass is emphasized slightly though which does make the mids slightly recessed and presentation warm but also has sparkle with the emphasized treble. It's a very fun to listen to earphone. The bass is just better with the UM3X and the presence more consistently warm with the UM3X. The Triple.Fi 10 does have that more V shape you seem to want though.
