The Protector is billed as the world’s first Balanced Portable amp. I actually know of no other balanced portable amp, so this is a reasonable claim. When used via its balanced output, The Protector phase-splits the input and amplifies the signal in balanced mode.
All of the below describes the amp IN BALANCED MODE, using the HifiMan HE-5 and the DT880/600. I did use the amp in single-ended mode, and in that regard it is still very good, but not quite as good as the P-51 Mustang. I would not recommend The Protector unless you plan to use it to drive balanced headphones.
Build Quality: A+: Typical RSA portable amp – about the best combination of excellent build quality and nice looks that you get No audible hiss whatsoever in low gain – in medium and high, there was some hiss at the very highest levels, but not enough to be an issue, since if you use that much gain you would be deaf soon anyway.
Treble: A+: A very slightly shelved down very top end, and a but very clean high end overall, and with outstanding detail. There is a natural quality to The Protector’s treble that is a little hard to describe unless you hear it. It’s effortless, grain-free, transparent, and smooth, while revealing lots of detail.
Midrange: A+: Mids were very slightly forward, although not grainy or unnatural sounding. And they were very transparent, and the overall the midrange of The Protector was astonishingly clean and transparent, to a degree that is normally reserved only to home amps. The acoustic guitar from Porcupine Tree’s “Stars Die” from “We Lost the Skyline” was extremely natural sounding, again in a way that is quite surprising.
Bass: A+: The Protector has weighty, deep, tight, and well controlled bass that is very well defined. RSA amps have all had a full, tight bass, and this is the case here too. It is hard to imagine wanting better bass out of a portable amp, or really any amp. The bass guitar and kickdrum intro on The Hush Sound’s “Medicine Man” was stunningly impactful.
Neutrality: A: Would be an A+ except for the slightly forward mids, which work a little better on some headphones than others. This is a much more subtle coloration than you get on most portable amps, but this amp has been aimed at the big boys, and in the context of that, it’s not more neutral, strictly speaking, than the other top tier headphones.
Soundstaging: A+: Soundstaging was truly a strength. Lateral image placement was very stable and well defined, and instruments floated in real space. Width and depth were absolutely among the very best if not the best there is in a portable amp, and again competitive in this regard with many home amps.
Transparency: A+: This also was excellent. The Protector is a highly transparent amp, to a simply amazing degree.
So used as a balanced amp, The Protector is a real little marvel. It’s capable of a VERY big sound, and makes portable balanced listening a real possibility. That said, The Protector uses a special connector, and so the user will need to either recable using it, or use an adapter from it to existing XLR balanced connectors (which is what I did). The latter is not all that practical for true portable use. So these issues do have to be considered, depending on what your use goals for The Protector are. There is no denying the sound is outstanding. Ergonomically, for me personally, the amp isn’t really useful other than as a battery-powered home amp, but if you have some high-end IEM’s you can recable for balanced, I can imagine The Protector would be just the ticket.