I received my Protector today. So that I could immediately listen to my balanced HD600 and HD650, Ray included an adapter. As of right now, the balanced HD600 and HD650 are the only headphones I have here that I can use with the Protector (in balanced mode).
Using my MacBook Pro-->Amarra-->Lavry DA11 DAC-->Cardas Clear XLR (female)-RCA adapter-->Cardas iLink mini-RCA-RCA cable-->Protector-->Sennheiser HD600 and Sennheiser HD650 (both balanced), I'm having some difficulty believing what I'm hearing.
At least where the HD600 and HD650 are concerned, this amp may dispel the commonly held (and generally true) notion that a portable headphone amp cannot drive these headphones like a good desktop amp. To my ears, with these headphones, that bridge has been crossed, and you're going to be reading similar comments from others a lot about this amp, as more and more people get their ears on it. Macro- and microdynamically, the Protector is desktop-sounding with the HD600/650--highly resolving, effortless, powerful, nuanced. (I've so far only listened to the Protector/HD600/HD650 combos in balanced mode--I'll try single-ended later.)
I listened to my JH Audio JH13 Pro and Ultimate Ears UE18 Pro with it just briefly (and single-ended only, as I have yet to order my balanced cables for those), and the initial impressions were excellent (again, just a very brief listen), and the channel balance is perfect.
Next up: Contact Whiplash Audio for Protector-terminated balanced cables for my JH13 Pro and UE18 Pro. If Moon Audio has the plugs, I may go for an HD800 cable terminated that way to see how the Protector does with the very picky HD800.
So far, this amp is sort of like a....a PortApache. And anyone who heard the balanced JH13 Pro/Apache system at RMAF will tell you how stunning that setup was. I'm hoping for some such magic done portably, when I get the JH and UE pieces wired up for balanced. We'll see, but this sounds promising so far.
I've been listening to this thing for hours--it's 4:23 a.m., I should be asleep, but I can't put it down.