Arthur, especially since the present sound is "truly fantastic", it's highly unlikely that the headphone output of the 3480 can be significantly improved with a separate headphone amplifier. There are a considerable number of fantasies floating around headphone forums on this topic, but many have a misconception about "cheap" headphone amplifiers added to receivers almost as an afterthought. Nearly all receiver headphone jacks are simply the same amplifier circuitry used to drive the speakers, but padded down to reduce the output to a level appropriate for headphones. Plugging into the headphone jack throws a switch which disconnects the speaker output and leads instead through the reduced headphone output.

The HE500s are about 38ohm impedance and have no unusual power requirements. If you're apparently getting ample volume without audible noise, there's nothing further to be improved. Enjoy.

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Enjoy the music, not the equipment.