I measured mine and can confirm the frequency response has the bass roll off focalpoint reported. People have improved bass response of the APPJ PA1502A headphone amp by changing the coupling and cathode bypass capacitors to larger ones. It looks like this amp uses almost the exact same parts, so I'm going to try the mods and see if they help. Hopefully the bass isn't just limited by the output transformers. In any case it's not a big deal if you're using a sub, you can just turn it up a little.