Upgrading 5.1 Surround setup for PC Use

[JXC]

Hi all! I’m new to the forum, I’m looking for advice in picking a sub and center channel to connect to my Harman/Kardon AVR 520 receiver. It can do 75 watts/channel. I’ll definitely be adding a pair of Klipsch R-51M bookshelf speakers for the front left and right channels, and will be moving my existing custom built bookshelf speakers(ported w/ tweeter + (probably) polypropylene woofer) to the rear right and left positions. As for center channel, I’ve got no idea what to go with, and my one requirement is that a somewhat heavy monitor has to be able to sit on top of it. I’m in a similar spot with the subwoofer, my existing sub is part of a Logitech speaker set and won’t be able to connect to the receiver, and it also has to be a powered sub as my receiver can’t power it on its own. My main use for the setup is listening to music, pretty much every genre short of hip-hop, rap, and country, my secondary use is gaming, only for single player games though, otherwise I use a headset. My motherboard uses Crystal Sound 3 if it matters, although I’m fairly sure it doesn’t so long as I’m using optical, which I will be.  Budget ignoring the R51s is about 400$, I’m willing to do slightly more if it could be worth it though.