For high signal level recording/digital source standard Realtek would likely be just enough for 600 ohms most of the time...
But for higher dynamic range content where average signal level is lower at scale with shorter high spikes problems would start.
Same for simply all around low level signal.
That Windows 100% volume setting just wouldn't be enough, because you can't get signal voltage high enough/you don't have amplification.
When DAC and output are integrated there shouldn't be any possibility of signal getting clipped unless designers screwed up.
And 600 ohm load surely won't draw too much current to overload output and cause clipping through that.
Exceeding output's current/power delivering ability happens with low ohm loads.
Just like why it's inadvisable to connect 4 ohm speakers into amplifier designed for 8 ohm speakers.
(at high volume setting you can even burn amplifier)
Clipping means that if scale of signal is from 0 to 10 and starting value is say 6 and you try to amplify it by two you'll get that maximum of 10 instead of correct 12.
It can happen both in digital part of signal chain with those "numbers" in editing...
Or in analog stage: Trying to amplify say 1 V signal by three while amplifier has only 2 V as max voltage.
Headphones should endure more power than your ears can handle without hearing damage starting in short time.
With any high power output/amplifier always plug in headphones only after turning volume to low settings.
Even with that 2.5X gain with O2 at high volume setting it wouldn't take much of input signal to drive lower ohm high sensitivity headphones to jet engine sound pressure level.
That comparison video is very good for telling headphone quality.
With bad headphones sense of direction is bad and there's no "depth": Everything sounds like coming from next to ear with some sounds just more muffled than others.
DT770 should have nice depth separation, like in that first minute gun shots coming clearly farther away with suddenly those bullet ricochets very close.
I've noticed that "Sonic radar" capable to giving graphical overlay on monitor for estimated/calculated sound source direction.
Would take that as indication how good Asus keeps their algorithms and their Chinese junk gaming headsets.
I mean with proper quality headphones fed by proper signal we can tell that lot faster just by listening...
With custom output stage design (kinda like in O2) Sound BlasterX AE-5 has possibly the best headphone amplifier in sound cards.
Software just looks way more bloated than that of Sound Blaster Z serie.
And for some more audio/electric engineering:
Creative's previous high end cards and better Asus cards have TPA6120 headphone amplifier chip which has unnecessarily high "slew rate" for audio and could do radio transmitter's job.
And likely because of that had datasheet specifying 10 ohm output resistor to keep it stable:
If having badly fitting load fast circuit/amplifier chip can become unstable and start to oscillate...
As car analogy just like some drivers doing too fast and hard steering wheel movements leading to losing control of car even in perfectly straight road.
While that output impedance can prevent such instability and also limit short circuit current protecting output that's only part of it.
Problem of output impedance is that if wanting technically the best signal headphone impedance should be 8 times output impedance.
That ratio is called as damping factor and basically tells how good control voltage source has to headphone driver to keep it following signal in all sitations.
Hence why Objective2 is designed with 0,5 ohm output impedance so that it drives even the lowest impedance/ohm headphones technically perfectly.
Anyway even 250 ohm Beyers don't care about output impedance of sound cards, while not needing as high voltage as 600 ohm versions.
I think this gentleman got it right:
"Man is a credulous animal, and must believe something; in the absence of good grounds for belief, he will be satisfied with bad ones."
-Bertrand Russell
And there's lots of money to be made by making people think they need £100 cables, £1000 amplifiers/DACs etc.
https://consumerist.com/2008/03/03/do-coat-hangers-sound-as-good-monster-cables/
Tom's Hardware actually did rather nice testing and again demonstarting why "audiophiles" don't accept blind testing.
http://www.tomshardware.com/reviews/high-end-pc-audio,3733-19.html
I sank $2000 of my own money into the DAC2 HGC last December, so I subjectively wanted it to sound better than everything else. Tests have shown that it doesn't. I was surprised, but, having been personally involved in the evaluation and believing in the integrity of what we set up, I rationally accept the findings.
Of course, we're ready for the audiophile community to rise up in arms about the statement you'll read next, but it's true that neither an intermediate enthusiast nor a serious one with ~$70,000 in gear at home were able to reliably tell apart any of the four devices once we properly set up a blind test with accurate volume-matching.
It had a weird hiss/buzz whenever the mic was in use which got annoying. 
