My hunch is that the Geshelli has a higher gain or different taper and is subjectively preferred because of level differences.
The sound difference descriptions perfectly match that of a somewhat 'louder' signal.
I have the same hunch. Also, it's interesting to hear people use descriptors like "punchy", "exciting" and "meaty". When I read that I think, "distortion". The stated THD on the Geshelli site is 0.01%. That is more than an order of magnitude higher than the Atom and means that distortion is 80 dB below the signal. That might be hitting a sweet spot where it's not quite registering as distortion to the listener but is giving the impression that the sound is more impactful than a truly transparent amp like the atom.
I would really like to see either measurements of the Archel or at least specifications from Geshelli on what output can be expected at 300 ohms. The Geshelli website only mentions 1W output at 16 ohms. The Atom measured higher than that at 32 Ohms. It is very hard to subjectively compare real output when both the low & high gain on the Atom are far lower than the fixed gain of the Archel. Has Geshelli published any output specs at higher impedances?
Some hand-wavy math. At lower impedances, the amp will hit some maximum current level, and the voltage and wattage will be correspondingly limited. At higher impedances current is no longer the limiting factor and it becomes voltage limited up to the maximum that the given op amp can handle at a given rails voltage, which in the case of the Atom is 8.68V rms at 600 Ohm. The Archel uses an LMH6321 for the output buffer which for a 1kOhm load has a peak output voltage of +/- 13.4 Vpp for a +/- 15V supply, which translates to 9.47V rms. So even if hypothetically the Archel could still output that voltage with a 600 Ohm load, we're looking at 150mW for the Archel vs 125mW for the Atom. Which translates into what, an extra half dB or so in real terms? It really shouldn't make any difference.
ATOM sounds right for low impedance headphones. But for ATH R70x (470ohms & 98 decibels) or other demanding headphones, it's getting dicey.
This is not my experience at all, and I suspect it again goes back to the linear vs audio taper of the volume pot. My main headphones are 600Ohm Beyer DT-880 and the Atom has always been plenty loud for my needs. For most listening I keep the volume between 11-12 o'clock in low gain mode, higher if it's classical or jazz or older pre-loudness wars rock, and I only use high gain mode if I'm doing some precision editing work on my own un-mastered recordings. The DT-880s are lower sensitivity (96dB) than the ATH-R70s, so even with the latter's impedance bump I would have thought it would be fine.