There seem to be a lot of defects with TE-BD21f version in Japan. Aviot's customer service seems to ignore customers, as seen from reviews on Amazon Japan.
I would say buy only from a seller you can return in case you have a problem with the device.
For your information, Amazon Japan does not allow to returns packages like electronics if they have been opened. (Unlike their US counterpart).
Agree. I ordered one pair from Tec_Japan from Amazon US. The delivery was pretty fast, only two days from Japan. Initially there was a noticeable hiss, though after burning-in it becomes kind of "cold hiss", meaning a noticeable hiss for the first couple of songs then un-noticeable. I contacted the seller about the hiss. They sent me an English manual, and instructed me to reset and test pairing other devices. When all did not work, they said "review the request and get back as soon as possible". No reply since then. I can stand with the "cold hiss", but if I got a defective set, I doubt the return/exchange could be painful.
That being said, I really love BD21f, even more so than their little brother, te-d01b. te-d01b still have the best mids to my ears, smooth, airy and detailed. But BD21f have the best package of TWS I have owned so far. The bass has excellent sub-mass extension, less amount than MW07 but almost the same texture. The mids is lukewarm between te-d01b/Nuarl Nt01x and MW07, the highs is sparkly and detailed. The timbre is very accurate. The advantage of the hybrids is clearly obvious that the sound is balanced with high-resolving details throughout the whole spectrum. In some songs they lack the airiness from te-d01b, partly due to the enhanced bass, but they're more versatile for any music genres I tried. The lukewarm sound sig also suits well with Amazon music HD, which I found very clinical from te-d01b.
They're also the most comfortable pair I have now, almost as comfortable as earin M2 I used to own. As much as I love te-d01b and nt01x, I always tend to grab bd21f. The isolation is also excellent. The other day I went to local Apple store to demo APP. To me ears, the overall isolation is even a little better than APP.
To me they're worth $259, though be aware the seller.