I don't think there's another IEM with similar bass like PFE 232.
The UE900 are aggressive, but the Heir sound harsher. The UE900 play directly into your face, but their upper mids/highs are so well done that it never sounds lacking or harsh. This is where they outperform the 4.Ai, IMO... and also flatter bass, so actually all-around more cohesive (YMMV). No idea where you get the v-shape from because if anything, most people are claiming the Logitech is warm.
Bass quantity will become noticeably less.
For your preference, I think you should consider an EQ. I also know somebody who loves his PFE but he uses the FiiO E17 with -2 dB on both ends. Not a dramatic change, but I agree that it sounds better that way.
I got the V-shape thing from a review a think. With that in my head, I did feel that the mids were less up front than on the 4.Ai. I'd be fine with the UE900 as long as the bass was as it is now, or a tad more. I'd probably prefer boosting the bass a bit on the UE's than cut frequencies on the high and low end of the Phonaks. I would have no problem EQing in theory, but I use a 7th gen iPod classic right now, and I don't see myself upgrading soon. I wouldn't be opposed to something like Rockbox, but the version for 7th gen iPod isn't far enough along for my taste. I bought an E11, but the noise floor on it kind of sucks. Does anyone have any other ideas to help bring up the bass a bit. Another cheap-ish portable amp with EQ or bass boost? The ZO?