Sorry for the long delay but work had most of my time occupied recently.
I would go for the HSU VTF-1 MK2, which is on sale right now for $419. It comes with port plugs and you can modify the sound to suit your needs. HSU makes wonderful sub woofers and you can audition them as well.
I don't think I can use this one since it does not have any speaker outputs in it. My PreSonus Audiobox USB external soundcard only has a R/L out that are going to my studio monitors. I'm going to have to run my L/R output from my external SC to the sub I buy, and then from the sub's outputs to the SM's.
...the SC does have a headphone out as well but I do not know if I can use that as a subwoofer output and keep all volumes controlled under one dial.
This is what I am working with...
PreSonus | AudioBox USB - Features.
How about a good pair of headphones then? $400 will buy you a heck of a pair of headphones and an amp/dac if you need one in your rig.
I do plan on getting a decent pair of 2.0 headphones in the future but don't want to spend more than $100 for them. Am open to suggestions.
I wouldn't think of 450 sq feet as a small room, by any means... What are the dimensions and ceiling height?
Emp has the 1010i and 10 on sale them may fit what you are looking to do...
My ceilings are about 9' high. The room itself is a very weird layout; like a tetris shape lol...imagine a red cross symbol but the west and south ends are connected at a diagonal angle. The north and east cut-outs are even proportions; my desk, speakers, and soon to have sub, sit on the inner-most left side of the northern wall inside that eastern cut-out...so somewhat in the center of the oddly shaped room. I hope that is descriptive and articulate enough to give a visual

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My floors are tile but I have hand-woven Persian wool rugs covering about 50% of the floor space. Walls are a mix of cinder block, drywall, and wood cabinets.
Regarding that Emp sub, I checked the dimensions and it is just a little too high @ 27 1/2" for the allocated locations I have set to place a sub unfortunately. I'm really working around space limitations in this room

. The sub has to go under my desk (25" H X 23" W but need room for my feet so no more than 15" D,) where my feet go or under my bed (14 1/2" H max for the sub there.)