If you are actually hearing a difference between the outboard DAC and Rotel DAC, then you probably have a computer setup issue, driver, or wiring issue with the soundcard. You aren't going to hear a difference between the Rotel DAC or any other DAC if all other variables are the same. Many dispute that and don't distinguish what seems to be a better design on paper and what actually is audible, but all well-designed double blind tests show otherwise (lots on hydrogenaudio site related to this topic). As an integrated curcuit designer of devices that include DACs, I'd love to say that all design attributes make an audible difference in sound, but that's just not true. Jitter is also blown way out of proportion in its audible impact, and I would be shocked if you or anyone else could prove jitter in anything made in the last 6 or 8 years was audible in music - no one else has yet proven any audible impact via a double-blind test. Digital signals were literally conceived and designed to maximize immunity to noise, so the whole "computer noise" and power supply noise worries are also overblown.
I suggest you look at your soundcard setup closer to give it a fair shot, and try to set up a blind test with a friend or two, to see if the outboard DAC is your best value compared to spending the money on more music or other needs. Most blind test fail on volume differences - the louder setup will be perceived as "better" - or fail to keep other variables the same across the test setups. To avoid software and processor issues from impacting the computer output in any setup, see if you are using ASIO or WASAPI drivers, and try not to run lots of other programs at the same time.
The good news is that there is far less to worry about than sellers of expensive equipment would have you believe, so have some fun with a fair test and use the test as an excuse to play a ridiculous amount and variety of music with friends!