small room questions

I use a room exactly the same size (and you will need to kill some reflections, for sure). It is NOT great but, at least, I have music. And, I'd suggest, that's all that matters. WAY too much analyzing going on by audiophiles. Just try to dig the music. After all, isn't Satisfaction and Revolution amazing music even in your car? By the way, I have not tried "diagonal".

Googling suboofer placement has a number of good sites.But always try to keep the crossover higher and the volume lower and move from their after long listening.People always want more but often less is moire.Might want to add Velodyne SCS-1 eq to smooth it out.If it's a real hard room it's counter intuitive but then putting some Sonex or even a nice rub behind the speakers on back wall can help (drapes other carpet if it's hard "live room").Sonex can real do a good job but it and other tiles are so pricey for what they are (friends used to just use the old 24 egg crates they get from supermarket for thier band practice rooms).For any room unless it's covered in shag carpet (like a Austin Powers "Shag Pad") can benefit from strip on sidewalls to stop standing wave "smearing" of tail end of note.Bob Harley in his "Complete Book To High End Audio" had a good suggestion.Get some roll out mirror mylar and put to sides of couch.Then hold lamp in your lap and where light hits mylar sound is just like a light wave and will hit in same place.A few feet of absorbent material will give sound focus.Other people like ASC tube traps or make their own.Others like the Michael Green triangle "pillows" (small) to put in ceiling corners.But given what a lot of this stuff costs look into DIY sites (Audioholics is good place)

All that said I have small room and 1)you can only produce so much bass in said room but the combo you have sounds right (20 Hrz. wave is close to 57 feet wide).My main thing with your room and mine is getting fare enough off back wall and placing speakers for good coupling and getting good imaging.Good you don't have other stuff in their but don't go nuts.Not much of this is helpful i.e. do this but if you have larger space you get to place spread and side walls distance and space to wall and seating the way you want but at some point those fine "Toys" would get swallowed up if too large.I did the Carpet behind (others make frame and put in odd foam) and treated sidewalls) but I know I'd rather large room and use panels or tube traps to tune it but have to work with what we have.Maybe grab a copy of the Harley book.
Let us know what you do and what works and doesn't
Cheers
Chazz

You can get awesome sound in a small room. I would keep the sub out til you get a reference. I know its hard to not use gear you paid for but you really should listen without it first. Play around with speaker poasitioning ALOT when (if) you bring the sub in, make sure it makes a real, seemless improvement.