Castle has even more than I do in there. For me it's just a few things: Measures flat low distortion low EMI (alternatively, good shielding to interference. I have had a couple phones and laptops - I go through a lot, and the Hifiman HM-101 that were very prone to interference, nothing else ever has been) Sufficiently low output impedance* *This number will depend on your headphones, I need really low because of my Merlins, which I love. But the original Hifiman line, and the first A&K AK100 had stupidly high, inexcusably high, output impedance, as do a few phones. But if you have a more standard impedance headphone, most portable sources today will have sufficiently low impedance for your needs. And that's pretty much it for me, until we get in to features. And the overriding feature for me is that it supports Google Play Music All Access, because I subscribe to that, and use Google casting for multi-room audio throughout my house. So for me, it's basically: Flat, low distortion, emi, and impedance, and runs Android. Unfortunately, my phone misses on the impedance thing.