As has been said before, with home theatre, it’s best to have speakers match when possible. The Martin Logan Axis (~$2300 for a pair) would be a good surround in-wall option to go with your Edge in-walls. If that’s too expensive or too large for the install location I’d definitely recommend sticking to Martin Logan models with their ribbon tweeters to keep the system sounding as cohesive as possible.
For a subwoofer, it’s not important to stick with the same brand, but the ML Dynamo line gets good reviews and has the benefit of built-in wireless capability (in the 700 & 1000 sizes) for placement flexibility. I’m guessing placement options might be an issue since you needed in-wall speakers...you can add wireless to any sub but that means buying an adapter module for most others. SVS makes subwoofers that are great quality for the money, offer a good return policy if you aren’t happy with how it sounds, and offer the option of buying "outlet" models to save a bit of cash... so I’d definitely recommend checking them out online before making a final decision. I have a smaller room than you, but my SB2000 has kept me very happy and is set well under 50% to keep my wife from complaining too much about how much it can shake the room. A room of your size with an open floor plan might ultimately benefit from a second subwoofer, but for now your money would be better spent on improved amplification, so don’t let somebody talk you into dual subwoofers just yet...though it is something to consider down the road.
Instead of a dedicated amp, I’d suggest starting off with a better AV receiver that includes pre-outs (so you can add a dedicated amp later) and has a better room correction system to address the fact that your room is going to create some challenges. Anthem MRX720 (~$2500) would be my first choice, but you could save a lot of money by instead buying the last models from Marantz or Denon now that their replacement models are out. Denon 4300H and 6300H are going for ~$800 & $1500, respectively, and will be a significant upgrade from your 910w. Marantz models along the same line are the SR6011 & SR7011 for ~$1000 & $1400, respectively.
So that would be:
Axis surrounds: ~$2300
ML Dynamo 1000: ~$1000
(or SVS of your choice at $???)
New Receiver: <$1500
Total: ~$4800
That’s without even considering buying anything used or refurbished. For someone who is "by no means an audiophile", this would likely be an awesome system but as you will read on here over and over, what you like is subjective and you never know for sure until you hear it in your own room.
Future upgrades could include a dedicated amplifier, second subwoofer, or additional speakers (to go 7.1, atmos/dts:x, or zone 2) without having to replace anything.
Just my 2 cents but hope it helps.
