Check this article out. It's not the most coherent thing I've ever read, and some of it's over my head, but it's certainly relevant here:
http://www.innerfidelity.com/content/musings-headphone-amplifier-output-impedance#1Qr4VCf6wDfMF5AZ.97
(don't forget to read the comments at the end)
One of the comments led to this adapter:
http://www.ebay.com/itm/Etymotic-ER4P-to-ER4S-6-35-to-3-5mm-resistor-adaptor-/282122706136?hash=item41afd2dcd8:g:VHwAAOSwEeFU5Xof
It looks like 3.5mm female to 6.35mm male. To me, that means once you select the impedance (51, 75, 100, 120, 240, or 300 ohm); that value adapter plus one 6.35mm-to-3.5mm adapter will allow you test this system w/any headphone w/TRS input jack & any amp w/TRS output jacks.
And that gets to something this topic has me thinking about. If the Atticus responds well (in some unspecified way) to amps having higher output impedance than SS amps (ie, OTL tube amps)--then I'm curious to know whether an otherwise excellent/reputable SS amp could be outfitted w/one of these imedance adapters -- and then have a fair contest vs a tube amp for powering the Atticus. In other words, if impedance matching is the "secret sauce" to getting good sound of the Atticus (rather than something inherently superior/different about tube vs SS amps), this setup would specifically test that hypothesis.