Pros: Silky smooth & accurate audio reproduction. Correct reproduction of complex classical passages
Cons: Small size but heavy
Over the past years I have had the opportunity to check out dozens of headphone amps. Several were decent but were challenged at higher frequencies. Some had some obvious deficiencies in design or execution. A handful were quite good. All struggled with increasingly complex classical music.
Based on Rudi0504's comments I made a special trip to Nakano (Tokyo) to check out the "Kojo Technology KM01 Brass" and the "Wagnus Epsilon S." I tried a few IEMs and headphones for both.
Both amps were superb. Both were different. This trip was well worth the time and effort. A brief comparison follows:
* The Wagnus is quite large but very lightweight physically. Clearly handbuilt but fit and finish was good. Wagnus had a bit more range in treble and bass; it had just the slightest amount of shrill that made it a bit more fatiguing. The demo Wagnus was really beat up and I think the volume pot was a bit scratchy; that could have been a/the contributing factor to the shrill. These are typical results of a cheap/defective/dirty/worn volume pot; but that might just be a characteristic of the unit. The volume pot is not cheap FYI. On hindsight, I should have asked to demo a new unit. I think I will follow up again on the Wagnus. Listening impressions - without going into detail, in challenging classical & some more modern popular music played at "red book" CD resolution, I found the Wagnus to reproduce music in a very agile manner with impressive clarity and extension both at the top and bottom end. It was not super-powerful. Very enjoyable.
* The Kojo is very small but heavy. The brass finish is not coated so looks stunning now but gets oxidized over time as the demo unit was not shiny. Fit and finish is superb. So smooth sounding. I think the highs and lows are just ever so slightly rolled off; maybe. Maybe not. For me, the Kojo combines the best characteristics of a tube amp and a solid-state amp. Almost no fatigue. I found the Kojo to be quite powerful. Very enjoyable and most musical portable amp I have found. Handles everything I send it with ease.
I purchased the Kojo and have been using it every day for a few hours. It takes one AA type battery which seems to last forever; well over a dozen hours.
Weight speculation - One obvious design aspect that differentiates the Kojo is the Brass casing, which makes it a heavy and dense unit. It does not come with rubber feet. I think this was an expensive and intentional decision by Kojo to improve sound quality.In my experience, the Kojo sounds best when placed directly on a hard surface (desk, hardwood floor, etc.). I have tried placing it on various isolation surfaces, all of which drained "life" out of the unit. So I suspect that the heavy brass case is one of the factors that make this amplifier perform well.
On a related note, some discussion on the impact of isolation/coupling can be found here: http://www.silentrunningaudio.com/
Also Kojo makes some other interesting stuff, including a ~$15k power regenerator for audio: http://kojo-tech.jp/
Edited and embellished from my comments on Rudi0504's appreciation thread