Awesome. Thank you for the diagram. That helps tremendously!
I'm afraid I have to go in for a bit of minor surgery tomorrow and I can't whip up a more detailed, to scale drawing for you at the moment. But I basically see only one good way to layout your theater in that room. At least, this is the way I would lay out my theater if that were my room
Make the wall that has Windows 2 & 3 your "front" wall. You can center the TV stand with the TV and all three front speakers on it between windows 2 & 3. Just get an oversized TV stand so that you have room on eithe side of the TV for the Front L & R speakers, or you could use a TV stand that basically perfectly fits the TV (around 48" wide) and then put the Front L & R speakers on speaker stand. You'd want stands that are about 30" tall for the HTM-200SE.
If you are worried about partially covering windows 2 & 3 with the TV, my suggestion would be to actually hand a curtain that goes across that entire wall. If you or your wife would still like to be able to have light coming in through windows 2 & 3, you could make the curtain essentially 3 parts so that you can draw them open to reveal windows 2 & 3, but still have the option to close the curtains so that you just have a clean, solid curtain look that covers that whole wall. I've done that before and had a curtain across the entire front wall behind the TV. It looks great! Trust me. Depending on the curtain you pick, it can even lend the room a real "theater" feel. There's just something about having a big curtain up front that makes it feel more like a theater
The couch would now go between the front door and the fireplace. This would allow you to have a nicely centered view of the TV, and give you pretty close to ideal placement of the front three speakers relative to the seating. With the dimensions that you've given, you could actually have the very back of the couch basically even with the right hand side of the front door. In other words, when you are coming into the house, you could open the front door, you'd have a clear path straight ahead of you to the fireplace, and the couch would be just to your right as you enter. It wouldn't block any walkways, and it should give you roughly about 8.5 or 9 feet from the TV to your head if you position the couch this way. If you stick with the plan for a 50" plasma, you could actually put the couch even closer to the TV and give the front door even more clearance.
You'd want to keep at least 2.5 feet clear on either side of the couch as a walkway. That should be no problem at all. I would highly recommend that you stick with just a regular 3 seat couch in this case, not a sectional or one of those 3 seat couches where one side is a chaise lounge (the "L" shaped type that you described). Instead, just stick with a nice, regular 3 seat couch and either an ottoman or a coffee table in front of it. I'd go for a convertible ottoman myself so that you can put your feet up, but also have the option to turn the ottoman into a coffee table whenever you'd like to.
So yeah, coming in from the kitchen, you would see the back of the couch, the fireplace on your left, the front door on your right, and the TV straight ahead of you in front of windows 2 & 3 with a nice, wall-to-wall curtain covering that front wall. If you really want the "L" shaped couch, you'd want the long, chaise lounge side to be on the right hand side of the couch. That way, you'd leave window 4 with better access for your future plans to convert that to a door to the computer room in the future
You'd have at least a good 6 feet behind the couch, which is nice. It would allow you to leave the computer where it is for the time being, the fireplace is not blocked by anything, and the front door and stairway will still have good clearance and an open walkway. You can get ideal placement of your front 3 speakers.
Just forget about the speaker wire that you've installed in the corners beside windows 2 & 3. You can have all of your A/V related gear up front in your new TV stand and just run the speaker wire and subwoofer cable straight from the Denon A/V receiver to your front 3 speakers and subwoofer. Heck, maybe one day you'll want to add "height" channels, and you can use the top corner speaker wire for that
I wouldn't worry about it though. I know you went to the effort of installing that speaker wire up there, but it's honestly better left unused. It's just in the wrong spot
On the other hand, the speaker wire runs that you've made to the position over the front door and in the corner above the computer might come in handy. You could use those for your surround speakers. Ideally, you would have the surround speakers directly to either side of your couch, so that would put them basically on the side walls in the space between the fireplace and window 4 and the space between the front door and window 1. Those would be the ideal positions for the surround speakers in this setup. And again, with the Denon A/V receiver in the TV stand in its new position in front of windows 2 & 3, there's really no need to worry about in-wall speaker wire. You can easily run speaker wire along the perimeter of your floor, coming out the back of the TV stand, running to the left and right across the wall with windows 2 & 3, coming back along the side walls with windows 1 and 4, and then just run it up the wall to your surround speakers in their ideal positions on the side walls directly to the sides of the couch. If you can't stand the idea of having that speaker wire running up the wall - or in the future, when window 4 becomes a doorway, you can no longer run the speaker wire along the floor - you can just use some paintable
flat wire from Axiom Audio or
FlatWire and totally hide that short wire run. The flat speaker wire is a bit expensive, but the best way to use it is to just run regular speaker wire that blends into your baseboard as far as you can, and then just transition to the flat speaker wire for any section that has to run up the wall. That keeps the cost down and it looks great!
So that's the way to lay out your room, IMO. That is certainly the way I would do it myself
Hope that helps!
And I'll try to create a drawing for you once I'm out of the hospital tomorrow. Hopefully, you'll be willing to forget all about the speaker wire that you already ran to the corners of the room. I think you're really being held back by wanting to make use of those. They're just not in the right spots at all! So maybe just imagine they're not there at all. Forget about them, and you'll be better off