Just to be clear, I am not suggesting you have hearing loss. I am only pointing out it is a possible explanation for your perceived excess bass in the U12t. It could also be a whole bunch of other things, such as insertion depth. If your tips don't fit properly, you have to push them in further. Don't forget that you are creating a Helmholtz resonator in your ear canal if your IEM's are vented. And the tuning of the Helmholtz resonator depends on the volume of the ear canal as well as the size of the vent. So, much experimentation is needed to choose the correct vent in your TIA equipped U12t's as well as careful eartip selection that both ensures correct seal and correct insertion depth.
This is why I don't think IEM's are a good choice for critical listening. There are too many variables and they are too inconsistent. For casual listening they are fine. I much prefer the sound of my DCA Stealths, and it is easier to get them to sound good. After the initial setup (a few EQ tweaks to my preference), I slap them on my head, and that's it. No need to keep fiddling with the IEM until it is seated properly before it sounds good.
I think IEM manuals should come with a warning about all the pitfalls of IEM ownership. You MUST select the correct tip, and it's not simply picking one that fits. After all, you can have a smaller eartip that fits, you just have to shove it in further. And a larger eartip might fit, but may not give a proper seal. I tried every tip that came with my IEM and I ordered aftermarket IEM tips before I found one that gave me the best sound. And because tip selection is so personal, there is no point telling you what worked for me. It's like telling you to buy size 11 shoes because I have size 11 shoes and they fit great. The only advice to give is to go try them.
Since you already own your IEM's, I suggest you keep playing with target curves on EQ until you find something you like. Then superimpose your corrections on published measurements of the U12T. This will be an educational exercise for you, because you might find that your preferred target curve is different to what is generally accepted. If it is different, then you have to ask yourself why, and what implications it might have for your mixes. I mean, if your personal preference for sound is different to the general population, you might be producing mixes that most people do not like. Remember - Harman target predicts preference for the general population. It does not predict your preference.
