Maaaan... 15k!? I'd have a filed trip!
Throw some info on your room dimensions and your sitting position. Gaming often implies moving closer to the screen for all the tiny details, movies, however, do not, nor does the music. This, obviously, depends on the screen dimensions.
If it's 80% gaming and movies, many here would tell you to think 'outstanding sub', preferably more than one, but only if matched with some pretty darn detailed mains.
On a very broad and general notion, people here advise not to spend more than one third on your electronics. With your budget you could go even less than one third and still have everything you could wish for. Get some real value speakers rather than fancy looking boxes. Speakers will bring real joy to your listening.
Even with a budget like yours, don't go crazy for 300$ decoupling rubber feet for the speakers or some Swarovsky encrusted cables. That's BS, no one here will tell you to spend a lot on these things. Don't think too much about separate DAC's, some esoteric electricity purifiers, some BS tube stages to be placed between your CD player and your Amp to make CD's play like vinyl. That's all just fecal matter.
According to your room, you will get some superb speaker set. This set will dictate the amount of power needed which will, again, be dictated by the amount of noise you like in the room you have.
Start with speakers that can deliver what you like in the amount you like. Go and audition speakers and always ask for a way to hear them at your place.
Describe the shape of your room here and state the specs, W/D/H, and people will have a much easier task helping you.
If you think your current mains might do a good job as satellites, keep them. You'll have even more room in your budget for the mains, center and subs.
I remember SVS having some sort of discount when buying matching two of their subs. It's worth checking.
Keep in mind; you can do more with two medium quality subs than one excellent.
