All this talk about the amount of bass a headphone can deliver makes me want to jump in with another factor to consider. I own both ES2's and UE-11's, previously owned ES500's and UM2's.
First, the difference in all frequencies in a custom vs. universal is ENORMOUS. Okay, that's been said before.
Next, while the phone is perhaps the most critical component of the system, it is in fact a system. All those upstream components make a very large difference, especially when we're talking about high end IEM's.
A couple months ago I switched from a 5.5g Ipod with ALO Jumbo Cryo to a 5.5g iMod with an ALO Jumbo Cryo X Silver cable. This was a huge difference! Speaking of the bass only, it got much bigger, much tighter, much more realistic. (Everything else got dramatically better as well.)
Source matters!
Since my UE-11's arrived I've been doing quite a bit of amp sampling. I plan on doing a more thorough review in a few weeks when I have some more burn in (and time), but in general I've found the UE-11's (with the above iMod) have much bigger bass with both the Tomahawk and SR-71 than other amps I've tried. I expect the Hornet is equally large, but I no longer own one so I'm just guessing. For the most part, I find the bass too exaggerated, maybe even bloated with RSA amps. You may like this combo quite a bit. It is smooth, rich and very full. I find it mostly a bit too rich and full. Not always and not with all music (the Rolling Stones sound really, really good with the SR-71), but mostly.
At the other end of what I've tried, the new Move is actually too light for me (only have about 20 hours on it). This might be a great choice for classical, jazz and acoustic music, but I don't listen to this much on my portable rig.
Right now my all time favorite is a PRII. I've read even more glowing reviews of the LISA III, but I don't have one and (probably) won't get one, it really is not portable. The iMod and PRII with the UE-11's are producing the best sound I've ever heard from any system ever, anyplace. (For reference my home system would cost in excess of $20k to replace, it is a poor second to what this portable setup delivers.)
My point is that with IEM's that are so incredibly good at resolving, the potential to resolve/emphasize/deliver any frequency is present. UE-11's can be too bassy, too shrill, to light, too heavy, too whatever, or unbelievably perfect, balanced, clear, transparant, neutral, or whatever. I guess I'd have to say that they can deliver whatever you want, but you need to spend the time and money (!) on the upstream components to get there.
Oh yeah, it's more than worth it when you do!