On to DACs...
Audio DACs are a cheap, silly little thing to spend any time worrying over. Much like Windows audio, that problem is already solved. Moore's law bludgeoned the DAC debate to death back in 2005 or so. DACs have become so cheap and so easy to do correctly that it is nigh on impossible to find a bad one anymore. Even the cheapest of DACs by any of the players that you'll find in PC hardware far surpass human audio capabilities.
Analogy time! Where is all the argument over video card DACs? The human visual system is a much higher bandwidth input system than our auditory sense. In the time between when we finally shifted to 24-bit color but were still outputting analog over VGA there was very little talk of DACs. Yes, card to card you could push resolution up high enough that you would hit the edge of any particular DACs capabilities. This was an obvious visual error though and easy to discern. It also went away by itself over time, beat to death by Moore's law. Plug any of your modern video cards VGA to your 1080p displays and it just works. The brand of DAC doesn't matter, even though said cheap little DAC is far higher bandwidth than any audio DAC you will ever come across.
The key word in the above paragraph is "obvious". Human audio is a lot looser about picking up errors. As such, we have a heavy tendency to decide an error exists when there is none, especially in a place like this where people with "magic ears" tend to gather.
Yes, PC analog audio can be done incorrectly. Most commonly, motherboards are laid out poorly in that after hitting the DAC the analog audio then runs alongside some relatively high-current, noisy supply traces for the CPU. This can result in all manner of buzzing and noise in your analog output. This means that your motherboard is designed in a broken fashion. It has nothing to do with the quality of the DAC.
You can take a $10 sound card with the exact same DAC on it, slap it in to a PCI slot, and it will sound perfect. Why? Because you moved those analog audio output lines away from other, electrically noisy lines.
Tomfoolery - Aside from a driver or software setup issue of one sort or another, this would also be a likely issue of yours. You've simply been merrily using your motherboard for a couple of years not realizing that it has has a crap layout for it's post-DAC audio traces from day one. Had you not put a sound card in the system, you would have known that it's audio layout was crap, then you would have returned the motherboard for a different design from possibly a different manufacture that actually functions correctly.
Don't blame the concept of onboard audio for your motherboard designer's poor decisions. That's like declaring that motherboard CPU sockets are a terrible idea because ASUS shipped you a board with a busted socket handle one time.