I just purchased your HA-160 headphone amplifier and am greatly enjoying it. I use it with two headphones, an AKG Q701 and a Sennheiser HD580. Both are great headphones but not easy to drive properly.
With the HA-160 I have had no difficulty getting more than enough volume. It has power to spare. Also the tone and detail make all my equipment sound much better.
I am very impressed with the quality construction of your headphone amplifier. It is built like a tank! I did some reviewing of the various heaphone amplifiers on the market and came to the conclusion that yours was the best value for the money and had excellent engineering and construction with quality parts behind it.
Congratulations on a job well done!