I finally got sick of comparing these to my Monoprice 8320s so it's time to tell the story:
vs Monoprice 8320
First impression is that the MT300 has boosted bass. This can be good for some, bad for others. Personally I enjoy feeling like I'm on stage, or in the room with the drums - it reminds me of my high school rock band days. But it's hard to ignore the "grand concert hall" feel of the 8320s. On switching back to the MT300s I feel like I lose a tiny little bit of the highs, and maybe some of their detail, though I'm not convinced. Soundstage closes in a touch too, but I feel it is still above average for IEMs. The mids are where I spent most of my time. The 8320 is such a clear, mid-centric earphone, it was hard for me to believe the TDKs could be that good, but they are. I think the TDKs have as much clarity in the mids as the 8320s and I don't get bored of them as quickly as the 8320s. After some time with the TDKs, and keeping in mind that I have other higher-end full-size cans, and IEMs, I don't feel like I'm missing anything with the TDKs. I think I just prefer the TDKs. There, I said it.
It's a close one though, and they easily complement each other more than compete, with the 8320s being really amazing for classical pieces in particular, and the TDKs as a general all-rounder. I mean, there are tracks where the TDKs can become annoying at first, but then the bass-head in me takes over and says "oh yeah, I play the bass, I'm the bassist, let's jam".
The deal killer for the 8320 is ergonomics. The awkward fitment and inconsistent seal of the 8320 makes it so much work to get fitted, that I sometimes pass on them because I want to get to my music sooner. The TDKs are easy to fit quickly, and if you do it fast enough, you can still feel the cool aluminum on your lobes.. few IEMs can do that!