Not really a fair comparison.
A 4K TV vs 1080p TV, the difference is obvious and can be seen.
Whereas 48kHz vs 192kHz difference is so little. Both can produce 0.0003% and below THD+N. There's Chinese saying, remove pants before farting.
48kHz DAC can achieve 0.0005% thd+n (and lower), can the speaker internal amp achieve below 0.005%?
If no, then why bother? The amp creates artifact in your audible hearing range but you are concern about higher frequency?
Composite amplifier do not work that well in clipping condition. The op amp will try to correct the clipping but we all know that's impossible to correct. :p
In my country when the AC mains is 50Hz, I will stress test at 60Hz.
A low z iem. You'll need an amp to drive that properly.
Balance output amp can produce more voltage, it may or may not produce enough current to drive properly; depending on the amp IC. A balance amp will each drive 5 ohm, most likely you'll see increase in distortion.
For a dongle to output 2Vrms, it's very suitable for high Z HP.
However most IEM users won't benefit from it.
For portability, dongles are suitable and best paired with IEM.
How about making the measurements into 2 tier.
1st tier: for dongles that are $50 and below:
Output: >=1Vrms
SINAD...
https://www.audiosciencereview.com/forum/index.php?threads/tanchjim-space-portable-headphone-adapter-review.46231/
I rate this poor. This device was suppose to drive headphone, which is why the 32ohm thd+n plot is the most important graph to read. Which it fail quite bad.
Higher than 0.01%...
https://audiosciencereview.com/forum/index.php?threads/i-think-i-found-the-perfect-usb-c-headphone-dongle.36914/post-1539367
It's not possible to get 2Vrms. Rest assure this dac amp can drive 8 ohm.