Just from my limited experience, the low gain output on the TU-882 will easily drive a low impedance IEM like my UE11pro customs too loudly for my listening purposes when the pot reaches 9 o'clock or so. For some, that would not make a very good amplifier, but I do have the volume output for foobar2K maxed out and could always dial that back to allow the amp to do more work with the input.
There is also a slight channel imbalance right at the low end, but it goes away when you go past that initial point. However, the UE11pro's sound great being powered by the TU-882 even under such non ideal settings, with more control of the bass than when compared with plugging them into my iPhone. I find myself plugging the UE11pros into the amp at home more and more often for listening to rock instead of my full size cans.
It's not that simple. Many cheap tube amps are OTL, so only work well with high impedance headphones. IEMs are very low impedance often and very sensitive. The Elekit is transformer coupled and has low-gain sockets for such headphones, which would explain why it works nicely with IEMs. When I eventually get around to getting some customs, I might pick one up, mod it, drop in some nice tubes and enjoy.
