I recently purchased a discontinued set of Hifiman HE500 headphones at a bargain price.I've owned a number of high quality headphones in the past such Sennheieser 650's,Beyer dynamics and Denons but was never impressed by their sound and always gravitated to my Axiom M80's.
At the present time the Hifimans are being powered by the headphone output of a Harman Kardon 3480 125 WPC RMS stereo receiver. The sound is truly fantastic to say the least.So much so that I find myself listening to them more often then I do to my Axiom M80 speakers.I'm just wondering whether I could further improve the sound by purchasing a quality dedicated headphone amp to power them.

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.