Pros: Fast, open, and dynamic
Cons: With a dry source, it may sound a bit dry
This is a good amplifier in my limited experience, *but*, it might not better than some of the leading solid-state competitors in this price range ($500-600). I haven't compared it too, for instance, Graham Slee, Burson Soloist SE, or Heed's Headamp. Also, you may want to think about tube amps in this price range, or seriously consider something from Audio-GD or Vioelectric.
I haven't got a lot of experience with components, so I'll explain what I have tested:
Beyerdynamic DT990 Pro
Grado 80 (old version)
Fiio e10 (using line out)
Denon CD Player DCD 860 (mid-1990s)
HRT Music Streamer HD (USB DAC)
Chord (on CD player and DAC)
Lows are very punchy and tight, so high quality in my opinion, though one might ask for more quantity for some genres.
Mids are detailed and rich.
Highs are detailed and clear as a bell. Well controlled and fast, so they are generally not fatiguing, unless your source and/or your headphones are fatiguing. If that is the case, this amp will not compensate for those problems.
I advise a well-balanced source and headphone, but if you have to go one way, I would tend to recommend a warmer source with generous bass, and darker headphones.
This amp doesn't ooze current like some out there (I think it may only have 100mw at 300 ohm, which is fine for the efficient German brands, but not for orthos), so it's fine for the headphones I've used. Output impedance is 5 Ohm, so best with headphones starting at about 32 ohm and going up, so while I can listen with my Shure 425 earphones, I wouldn't recommend this amp for them because of the high noise floor.
I haven't changed the gain, but it does have a high gain jumper. The pre-amp option isn't very good, in my experience. Maybe someone more tech-savvy could get it to work better.
A curious thing is that with my Denon CD player, there was very little difference between the headphone jack of that CD player, which is quite good, and the Rhinelander. The Rhinelander actually sounded a bit worse, in some ways, with a dryer mid-range. This is an old CD player, but it sounds very, very, good relative to it's current market value (about $100). The space between instruments is very wide, even if the actual presentation thus becomes a tiny bit unrealistic. The mids are good, and the lows are rich. However, the highs are not very realistic or accurate, all of a sudden becoming slightly grainy and tizzy. The Rhinelander doesn't really know how to handle this combination very well. It seems to prefer a lot of high-end detail to shine. All of that said: if you're really on a budget and listen to CDs, you may want to look for an old Denon or Marantz player that has a headphone jack: they're quiet and very good. However, if you listen from a computer, you probably do want separate DAC and headphone amp components.
So, when hooked up to the Music Streamer HD it sounds amazing: punchy detailed and full. Also, it sounds much better than my vintage Onkyo TX-330's headphone jack. Though the vintage receiver adds even more body to bass and mids, it's all fat and very little muscle. I was relieved to go back to the Rhinelander.
This little guy is a muscle machine. Kind of like a Jack Russel Terrior of amps. Cute. Fast. Smart. Lean and strong.
Or if you prefer a different metaphor: it's a clean and crisp traditional German lager.