I bought a pyle PHA40 headphone amp. It appears to be close in size and specs to the Behringer HA400. It powers off the same type of cable used by the KX2/KX3, same voltage and polarity. I paid 16.60 for mine on Amazon. I connected it to my KX3 and the audio quality sounds good. I didn't detect any extra noise introduced by the amp. 73 Len