Also want to know it. And meanwhile I am going to make a total newbie question. Hope to not offend anyone...
Whilst the physical tone volumeters produces some noise (deppending on many factors like the quality of the implementations and so on...), does the digital equalization and tone adjusting on the built-in amp/DAC produces noise?
I imagine it doesn't, but want to be sure.
In my experience noise (that you actually hear) is really around the amplification gain that is applied, though I imagine a cheap DAC will introduce noise (after the digital EQ) which will then be amplified and be horrible.
So Amp vendors what to have good specs as far as output watts but unless the chip has lots power (like the TPA3255) they will tempted to up the gain to make their Amp sound much louder. This is definitely the case with some of the older TPA3116 Amp I had tried, but this Aiyima A01Pro was actually pretty good, no near field audible noise (i.e. put on a very quiet track and put your ear close to the speaker).
I haven't heard the SMSL DA-6 but the reviews aren't great on amazon (probably why it's $49) but looking at the raw data - its not particular powerful e..g. https://www.mouser.ie/new/infineon/infineon-ma12070p-audio-amplifier/
I especially don't like the "2×80W peak output power (26V PVDD, RL = 4Ω, 10% THD+N level)" - Nobody wants to hear 10% THD+N as that would be horrible. So I would avoid that SMSL.
There is another digital amplifier that quiet like - another Aiyima the D03 https://www.amazon.com/AIYIMA-D03-Bluetooth-Amplifier-Amplificador/dp/B098QSPJJG/ref=sr_1_3
- this has some digital EQ (though not as fancy as the SMSL) even some digital crossover filter related to the subwoofer output. - though I believe some people had some reliability problems. I would hope that Aiyima have ironed out the issues in the manufacturing process.