You will most certainly have a higher signal to noise ratio than with the on-board soundcard of your mac, meaning that even very soft signals won't be noisy because of the interface. If there is any noise at all, it's most likely already in the recording.

You should absolutely use your Apogee. Many onboard sound cards also will not support 24-bit audio. And you'll wonder what is horribly wrong? Nothing. And so the output of your Apogee should feed your powered monitors in a best case scenario. I only recommend mixing on headphones when there are no other ways to monitor on speakers. Headphone mixes rarely sound good through speakers. That will only happen after you learn what your perceptual differences are between a good pair of quality monitor speakers and how it sounds in the headphones. Which will be totally different I assure you. And so there are those of us that choose our headphones that we feel most closely approximates our listening experience through our preferred monitor speakers. But if you can, always try to mix through monitor speakers and check your mixes through headphones. Not the other way around unless it's 100% necessary to do so.

JBL KRK & Sennheiser monitoring done here.
Mx. Remy Ann David

thanks!