I have some powered Sony desktop speakers that are a step up in price bracket from yours (though I haven't used them at all in the past 3 months), almost universally the headphone jack on these external speakers is cheap and noisy... It should be a little better directly plugged into the X58, but even so the amps built into soundcards aren't very clear and also have to contend with even more interference from other electrical parts.
I like the Omni because it's basically an external soundcard. It offloads the sound processing load from the CPU (more resources for other stuff), has a better DAC and Amp than what's built into any motherboard, and easily allows you to connect to upgraded amps or DACs later. It will also be the hub to let you connect headphones and up to a 5.1 speaker setup, and switch outputs in software without having to unplug anything. I like the virtual headphone processing very much.
A hi-fi amp will still improve on the sound, but my K712 sounded very good plugged directly into my Omni.