Thanks to Nick_N for tipping me off to these iems.
I plugged them into my ears right after I picked them up before Christmas. The bass sounded a little flabby so I burned them in for 30 hours.
A little patience paid off. The lower registers definitely dominate, but they a nice timbre and texture. I was listening to a movie and had to rewind it so I could re-listen to a short section of the soundtrack. The bowed cello sounded so authentic to me. Now I remember the music, but not the movie.
The frequency response isn't U shaped, but it gradually drops off as it moves from the bottom end to the top. That's pretty clear when I listen to log sweeps. Nonetheless, the mids are pleasant. Some female vocalists that sound thin or dry on other earphones sound pretty good with the TDKs. The cymbals are there, but they don't offer much sparkle.
I ran a few test tones and, boy, is the sub-bass there in spades. The lowest track I have is 16 Hz and it was audible and uniform — no crackle or buzz. (And I thought you felt sub-bass more than you heard it. I guess I learned something new.)
Overall, I think I have a preference for the brighter Monoprice 8320. But I've been swamping between the TDKs and Monoprice units pretty constantly since Christmas. (The TDKs are far more comfortable and stay in my ears much better, even when I'm active.) And for a total of $15, I got them both, including shipping on the Monoprice units. You can't beat that.