If your happy, that's all that matters. With the quality of current on-board sound, I wouldn't recommend a sound-card unless you want a headphone amp and prefer something internal. My Asus Essence STX sounds the exact same as my 4 year old on-board except being able to power my 250ohm DT770's.
I'm actually using my onboard for input because the STX adds an insane amount of EMI. Keep in mind I bought the cheapest X58 board at the time of launch, the non-cutdown boards have an even better Realtec codec.
If you speakers accept S/PDIF, definitely use that as you want to have the DAC as close to the end of the change as possible. Unless that DAC is absolutely terrible for some reason.
*edit*
For giggles I plugged my current headphones into my onboard for the first time. For the most part it sounds decent except not having any lowend and not being able to ramp the volume up very loudly. For sober listening on an easier-to-drive Samson SR850s this wasn't too big of a deal. The occasional quiet youtube video or "having the whirlygigs" lead to myself wanting a headphone amp.