This and That:
I really like the design; good build quality. Comes with a carrying pouch that I am using to store the DAC in when I don't have it in use (my main field of application for the Geek Out IEM 100 are headphone frequency response measurements).
When in use, there are some white, unlabelled LEDs on the top. The sticker on the rear gives an explanation (they're sampling rate indicators, however since the firmware was changed, some of them also indicate which of the two digital filters is active).
It seems like the DAC used to feature additional Crossfeed and that its side buttons were used to control the volume, however the Crossfeed and volume control feature were unfortunately removed in a later firmware update. Now the two buttons let you only select between two digital filters.
Additional drivers need to be installed on a Windows computer. That goes along with the Light Harmonic Control that is mandatory for volume control since the Windows-internal sound mixer is way too imprecise. This method of volume control is frankly a bit tedious and I really don't see why LH Labs removed the seemingly more convenient volume control through the buttons. (Perhaps some additional software like "Volume Step Adjuster" would be a great workaround, but I haven't really tried it yet as I'm primarily using the DAC as a low impedance audio output for in-ear and headphone frequency response measurements.)
Sound:
The DAC has got two headphone outputs, one labelled with 0.47 Ohms and one labelled with 47 Ohms. While the former isn't true (it's even lower), I haven't specifically measured the latter, however the output labelled "47 Ohms" has definitely got a high output impedance. Having two headphone outputs with very low respectively high output impedance can be quite convenient when someone wants to see if/how an in-ear's/headphone's frequency response gets skewed by a high output impedance.
Frequency Response (no Load):
The two digital filters, fast and slow roll-off, are shown. Not much surprisingly, they act just like they should and the frequency response is also just as flat as it should be.
Output Impedance (Load: Ultimate Ears Triple.Fi 10).
The measurement was performed using the fast roll-off filter (the other one performs accordingly). The tiny deviation leads to a calculated output impedance of only 0.1 Ohms, which is perfectly low for all in-ears.
Hiss Performance:
Using near-extremely sensitive in-ears such as my Shure SE846, there is little audible hiss. So still on the quiet side and among the clearly better devices, but not as quiet as my RME ADI-2 DAC, Leckerton UHA-6S.MKII or iBasso DX90.
Subjective Listening Impressions:
Just as clean, transparent and neutral as pretty much most other well-constructed modern audio devices. No hints of "glare" or whatsoever.
Perhaps the impression of a slightly smaller soundstage using sensitive in-ears, but none of that using less sensitive in-ears and full-sized headphones. Just a nuance anyway, if it's indeed even present after all and no assumption.
Conclusion:
Perfectly low output impedance, still good hiss performance (but not fully flagship-level hiss performance using near-extremely/extremely sensitive in-ears).