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Hi Folks,
Think of the BHA-1 as you would amplifiers in a normal speaker setup. Depending on the speakers you have - their efficiency, power handling, impedance, room size, levels required etc. - your choice of amplifier would depend heavily on the forgoing requirements.
Same with the headphones chosen - are the phones difficult to drive - have very low efficiency, limited power handling etc. So the headphone section in a preamp typically is an IC chip (BP26) and is generally limited to phones with impedances above 50 ohms and reasonably high efficiency. The BHA-1 though is a fully discrete Class A circuit which has huge voltage and current output capability so driving low impedance phones with high power handling and low efficiency is where it shines. Choosing the correct headphone amp is entirely dependent on the phones chosen - same as in a normal speaker and amplifier setup.
Hope this helps.
james