Someone want to talk me through USB DACs

as someone who has .. well, "dabbled" in audio engineering, i am always puzzled at these questions. I've not had any complaints about the quality of my PC onboard audio output since, well, ALC883, years ago. Plug pinjack cable in PC, plug same cable in amp, play through speakers.

Never had any cracking, hissing or noise. Yeah sure my current Logitech desktop speakers can crack a bit if the pinjack is inserted "wrong", but fiddling with it will fix it so there is no noise.

"HIGH-END" is a dangerous concept that should be abandoned. It means losing grasp on the fleeting nature of music and of our enjoyment. Stop measuring the hiss with professional tools and pump some AQUA loud enough to make the walls shake.

I've been through this a bit, I used to run analog audio out to a Yamaha mixer, and I always had to tune in the noise floor. Always seemed like a grounding issue. Fast forward to my current setup, I run a Zoom mixer with that also has a USB DAC, and interestingly enough I get noise on the mixer UNLESS I have it plugged in to my computer.

I am barely knowledgeable in EE but if I had to guess, your DAC has some sort of isolation stuff going on. As in, how the circuits are actually routing to ground is different with the gain button. As for the monitor USB port not having noise, that makes me wonder if the monitor itself also has some kind of grounding isolation going on. As in, the monitor seems to have some kind of isolation from the PC, and the noise is coming from the monitor, so it helps to be grounded to that instead of the PC.

But I don't really know tbh, once I found a config that was noiseless I stopped investigating.

I have dabbled ina lot of DAC's.
Here is my honest take.
If its cheap, most likely the one inside your gaming motherboard is several leagues better.
If its a expensive one or one from like Drop, or SMSL at the lowest, it will be a pretty OK DAC, but u still need a headphone AMP to pair it.

Most of the line noise is probably from your PC or other stuff plugged into the same rail on your main house breaker.
You will need a noise isolation on that plug to clean it up if thats the case, but that is all a hit or miss.

I rarely use my expensive dac and THX headphone amp, because my PC is enough for most things.
Sure if i want to play those FLAC audio at high vol, on my expensive as heck Focal Headphones, then i will use my DAC with the THX Amp.

Otherwise for 99% of the things i do, i will just use my onboard sound.
If i need non headphone sounds, i use the optical on it paired with a Speaker bar.
If i need to impress the neighborhood, then i'll just turn on my Marantz Cinema amplifier and the 9 million speakers its connected to, and simulate an earthquake from its dual 12inch sub woofers.
(the most regretful purchase i made... not because it sounds bad... but because it was a total binge buy and audiophile stuff ISNT CHEAP, and id say its more expensive then even the TOP RGB GAMER GEAR all combined.)