Thanks. I also have a smsl m300se, which has an actual headphones amplifier. My daughter took over the m300se because it takes bluetooth as an input.By doing it this way, you're most likely boosting the Zero's bass response a lot.
Nothing wrong with that if it suits your preference, but something to be aware of nonetheless.
The SU-1 is designed exclusively for plugging into external Amps.
If you're looking for a compact DAC that's actually built to drive IEMs directly, then take a look at the Topping DX1.
That one will reproduce the Zero's tuning as intended by the designer.
Edit: Sorry, my mind jumped to the Truthear Zero. It's the Truthear Zero whose bass will be boosted when used directly from Line out.
The 7Hz Zero OTOH should stay pretty stable WRT frequency response.
Whether the SU-1 can cope with the increased current draw due to a load which it wasn't designed for, is another question.
The Zero:2 are very sensitive. The SU-1 drives them more than adequately, at least from a volume standpoint. I'm listening to this setup right now and any volume above 40/100 in the PC is too loud for my taste. Anything above 50% is probably not safe for my ears. The bass sounds excellent, not overly boosted. Sound is extremely transparent. I will report if I blow up the SU-1, but I doubt it at this point since I've using the setup for hours a day for a few weeks.