gabrieluko, although you might think that you want to connect your headphones to the RCA outputs, you really don't want to do that. Those are line level outputs and are not designed to load into 8 ohm headphones. Those outputs want to see 10,000 ohms or higher, not 8. It will cause clipping and distortion and you might even blowout the output section of your audio interface? And you wouldn't want to do that now would you?

You need a headphone amplifier, plain and simple. Which really means any amplifier under 5 Watts, since 1 Watt can blow your brains out, if directly coupled. So any of those commercially available 1U rackmount headphone amplifiers will provide you with what you need.

Smarter is better
Ms. Remy Ann David

Understanding Headphone Power Requirements
http://www.rane.com/hc6hp.html

I use more than 1w amps ... but it's not about the power but about the voltage headroom

the power required can vary so much, depending on
the impedance
and the sensitivity