This may help. Check out these two guides on speaker placement: http://www.audioholics.com/home-theater-connection/speaker-placement-setup-tips-for-upgraded-home-theater-systems
http://www.audioholics.com/home-theater-connection/home-theater-speaker-layout-an-essential-guide
The advice for a stereo setup is the same even if you don't add in the other speakers. Figuring out where your speakers will go will help you to make some decisions.
So if you are setting up your speakers for use with your TV and Xbox, if movie and gaming use is important, then you definitely want a sub. Lots of sub bass content in movies and games. And in movies, lots of 20 to 40hz content that towers would never reproduce. And if movie watching and gaming is important to you, then you might want to think about a surround system.
Asking whether a sub will muddy up the sound is almost like asking if a hammer is a poor choice over your shoe for hammering in nails. A sub is THE specialized tool in home audio designed for producing low frequency content below 80hz. Unfortunately, most people have only ever heard cheap subs that are boomy, with distortion, and that are poorly integrated into a room. If you like bass heavy music like EDM, dubstep, reggae, hip hop, or rap, a sub is a necessity for best sound. I recommend 12" subs from SVS Sound. They are an Internet direct subwoofer vendor that provides such good price/performance values that they would sell twice as much or more if they were at brick and mortar store. A good sub from them will reproduce the bass in movies accurately, more like a good movie theater (and even better than a cheap theater).
I have not heard the Oppo, but keep in mind that there are greatly diminishing returns as you spend more on electronics. On the other hand, a home audio speaker system (particularly HT setup) is much more expensive to achieve the same level of sound quality as a good pair of headphones. So it sort of depends on what your priorities are. Then think about your total budget you have to spend on everything and prioritize it. Could be that a mid level Audio-GD DAC/amp would be the way to go for everything you are trying to do so that you can put more money toward something else.
So more questions:
What are your primary headphones you want to use with a new desktop dac/headphone amp?
What are your priorities here?
Is gaming and movies important with your setup? Do you want a surround sound setup?
Do you like bassy music?
Do you want a really good sounding room audio system?
What is your total budget for everything?
Also what country do you live in?
Then go from here