I think you might be disappointed in your pursuit here, not because of the gear but because of room acoustics. It's the most overlooked issue with speaker setups, and can make $2000 speakers sound like junk.
That crisp, immersive sound from headphones occurs because the only thing the sound is interacting with are the cups and your ears.
In a small untreated room, you're going to get all kinds of reflections, standing waves and, if you have speakers close to the wall, boundary interference. The effect is likely to be pretty muddy.
Believe it or not, you can probably get more bang for your buck focusing on finding the sweet spot for your two existing speakers. When it comes to soundstage, no headphones can compare to two properly set up speakers. That sense of being surrounded by instruments or that feeling like "somebody made a sound in the room" can be really freaky with just two well placed speakers.
All that said, have fun with it. You'll learn way more by messing around with those extra speakers. Just don't make the very common (and costly) mistake of thinking that your speakers are lousy when the problem is a room filled with reflective surfaces.