Two part question:
Do you guys have any cheap solutions to improve the acoustic treatment of a room? I have a few photos and things on the wall, bookcases around the room, and rugs on the floor.
Secondly and more importantly, I have a full wall that is mirrored (like a sliding door closet) that is to the right of my speaker setup. I currently have a curtain hanging over it, but figured there was a better solution.
Any advice?
Bass traps in the corner will help, and acoustic panels on empty spaces of the wall can help. Having a large couch might also help or something like that
The mirror issue is kinda hard to address, but putting a super thick comforter on there is probably the best you will get for that
The curtain is probably not a bad start and might be as good as you can get.
A lot depends on how controlled the listening environment is. If it’s really controlled, always listening from the same spot I’d measure it and spot treat.
Is it a 5.1 system or Stereo?
Maybe a porn studio/site could help?.. joshing just joshing 
This may sound stupid and a bit overkill, especially since I have giant RP-280FA floorstanders, but the room my setup is… my bedroom.
Yes, I have a surround Atmos setup around my bed and will never have to go to the theater again, lol.
This also means that couches won’t fit.
Ok, so you have a bed in there right? That would probably do the same thing as having a couch in the room
Also that sounds like a pretty sweet setup lol
I would say just doing the aforementioned things and potentially isolating the floor standers
Bookshelves with stuff on them are great acoustic treatment. Especially behind the speakers / instruments.
What do you mean about isolating the floorstanders?
Also, since it’s my bedroom, the speakers are a bit close to my back wall. I don’t have much space to pull them out.
Should I put panels on the back wall? Stuff the back ports?
I would perhaps put some panels behind the speakers and the back wall, and perhaps put some isolation pads or pucks under the speakers
I have a throw rug underneath each speaker. Is that good enough or will pucks be better? If so, any product recommendations?
That should be fine imo
My thought would be to try to turn it into a diffuser and still keep a lot of its mirror functions as well: by sticking something like these kinds of floating shelving units to its surface: http://i.ebayimg.com/images/i/361476577888-0-1/s-l1000.jpg
You’d end up with it looking something like this: https://cdn.notonthehighstreet.com/system/product_images/images/001/318/702/original_decorative-wooden-honeycomb-mirror.jpg
The more irregular the shapes the better the acoustic diffusion effect though, so you wouldn’t exactly want a honeycomb or anything like that.
But it could be hard to get enough of them for good coverage at the same price of the curtain you already have = free. 
