i was in a similar position to you a few years ago. after an extensive audition period with a number of canalphones, i most enjoyed the sound of the ER-4, though like many others, wished it had a bit more in the lower frequencies. i also wanted something that didn't stick out from my ears as much, so figured that i would get one of the custom IEMs.
at the time, the two major choices were Ultimate Ears and Sensaphonics, and with the latter being local to me, i went in to hear a demo pair of their custom phone. it certainly had amazing bass, but still had a bit of high end roll off, and the overall sound was a bit unnatural. so, after exchanging many PMs with members here, a couple of which had purchased custom from both UE and Sensa, i decided to order the UE-10 Pro, and i'm glad i did. they retain a similar sound to the ER-4, but fill in a bit at the low end, and manage to sound overall a bit smoother, without losing detail. while they do sound good straight out of a portable source, they will benefit from an amp.
when considering the UE-11, keep in mind that just because it's more expensive, it's not necessarily "better" than, or the successor to, the UE-10. it was designed/tuned to have a different sound signature, which is much more pronounced at the low end. i think the UE-10 Pro is the best choice if you want something close to the ER-4 sound (overall neutral).
however, in your case, you shouldn't have to rely on opinions and guesses, as you say you live close to UE's headquarters. i'm pretty sure they have universal fit demos for all of their custom phones, and you can hear for yourself which sound signature is best for you.
cheers,
phil
