Best components to optimize Martin Logan 50XT and Edge in walls

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.

I agree with auxinput that its possible that you just don't love a warm sounding AVR/amp, but its also possible that you'll enjoy a higher quality and more powerful one just fine...you'll only know for sure once you've tried it. Look for somewhere that will let you audition the equipment or at least has a good return policy. The Anthem MRX720 (~$2500) is great and will probably be my next AVR. I haven't heard the Cambridge (~$2000) myself but the reviews are fantastic and it is largely what has kept me from buying the Anthem already. The Cambridge only has pre-outs for front R&L channels, doesn't have HDR nor Atmos/DTS:X compatibility, and doesn't have room correction built in...none of which may be important to you at all, but are things to be aware of. They both probably sound better than what I suggested but also cost more so it will be a matter of balancing what matters most for you and your budget. If you spend less on the surround speakers or subwoofer than I outlined above, you can definitely make either work within your budget and they'd be great.

Once you've cut holes in the wall and installed speakers, convincing your wife that you want bigger holes / bigger speakers might not be an easy sell...that being said, big holes for big surround speakers might not be a battle worth fighting at all when it is an area where there is definitely plenty of room to save money vs the ML Axis I mentioned above. I just went with something close to your Edge speakers and tried to make the rest of the budget work as a starting point to consider...I have no personal experience with ML's in-wall speakers so can't tell you much other than keeping a similar profile with what you have is at worst not a bad idea.

Personally, I'd suggest not starting off with both a new receiver AND power amp. You're right that a power amp is more useful long term since the pre/pro is what tends to change more quickly, but if you get a really good AVR, it may simply not be necessary. You're also likely better off getting a good quality AVR than cheap separates. Good separate pre/pro & power amp (i.e. Emotiva XMC-1 @$2500 & XPA-5 @$1600 for a reasonably priced example) are going to be tight with your budget, especially since you have other needs and would start having to buy interconnect cables. A good AVR is less expensive and you can always add an amp for any channels that have pre-out connections down the road while still using the AVR as a pre/pro and as an amp for some of the channels or zone2 functions... later replacing the AVR completely with a better pre/pro if you feel the need. If you're interested in used options, that makes separates more feasible, but not sure about your feelings on buying used.