Wow this got quite long, but here you go:

I own a FiiO K5 pro and the Xduoo TA-26 and use both, but to be honest here, my onboard audio can handle the HD 6XX just fine. I tested it a few times and I really would love to say it was worse, but it was honestly fine and, as far as I remember, not much different.

I've yet to plug my headphones in some amp-dac setup and say: "wow, what a difference!", so there's that.

My setup is more for looks and the feeling it gives me. Warm, slightly glowing tubes, dedicated volume knobs, all of that is very nice, but I'd be lying if I would tell you that these amps blew me away in comparison (the TA-26 is just solid state sound that's slightly fuller, like 10% difference at best). I also believe that dacs barely differ from each other if their goal is a neutral soundprofile. The FiiO works just as well as a 1k dac I've tried at a friend's place. He was quite shocked, which I found very entertaining lol.

But take this with a sprinkle of salt. I am sure that not every onboard audio solution is as good as dedicated dacs and amps. It's just my motherboard that works fine and others might not. My phone with the EU Samsung dongle doesn't get loud enough and the bass is actually reduced and my Nintendo Switch also seemed to be lacking in some aspects.

In the end it's up to trying things out. A and B testing with matched volume, finding out by yourself if there's a need for amplification. Heck if money isn't an issue then go nuts, it's a great flex, but on the other hand there are many on here that want the best bang for their buck and that's totally fine.

So we should stop lying and tell them the truth: it always depends if you actually need thing xy, so try before you make your wallet cry. Heck even I'm a little bit biased because all of this stuff I purchased was expensive for me. But I am really trying to swallow my pride here and tell you what I've actually experienced.

As and example: my last few weeks consisted of swapping tubes like some lunatic only to be sitting there, sad and alone in my room, trying to deal with the fact that there's barely any perceivable difference between all of them.

TL;DR: I sometimes wish some devices made as big of a difference as some people claim. In the end you have to try things by yourself and decide. I personally wasted some money (on tube rolling) that could've been used for something else.

Edit: oh btw, I didn't talk about things like hissing sounds from some aux ports or USB ports picking up GPU sounds and such. Some dac-amp solutions might help here, especially optical cables (these fixed the issue of my tube amp picking up random noise from my PC).