850 lor... catch is no warranty. New stock, old model.

I will just use the onboard unless it sound very obviously bad. My onboard Realtek ALC1220 sound quite ok TBH.
Still a high quality DAC will be better than onboard, only that I don't know how good is the S730 to hear to difference.
I do run my AegoM from a Schiit Modi DAC for music, but I will use the onboard for gaming (7.1 + mic).
Does your onboard got a Optical out, because S730 supports optical/coaxial maybe can also try that?

Get an external DAC. It might not necessarily sound better, but will make your heart feel better
Plus, if you get DAC with headphone amp, you can use it with headphone. Especially useful if it have volume knob.

S730 have coaxial and optical input, if your mobo sound card got this output you can use it to send digital signal to S730