I'd go with better room treatment and your turntable. Consider corner bass traps from Gik or ATS. Check the AM Acoustics room mode simulator to help you place your speakers and listening room away from the lowest modes.
Honestly, the order should always be treat the room first, then decide about a sub. In addition to bass traps, reducing the overall reflectivity of the room can make your speaker's bass really come out.
@erik_squires I appreciate the input! I actually have done GIK bass traps in the room previously. Even went as far as paying premiums for specific fabric choices to help appease my wife's aesthetic requests and get something that would work in the room long term. And after about a year, I finally put them up for sale. They didn't have an impact on the sound, they were visually unattractive, honestly I was disappointed in the build quality and attention to detail from GIK, and overall walked away loosing a few hundred out of pocket and a feeling of "at least I gave them a try."
Now I'm not anti-room treatments by any means. They are simply just challenging. It's hard to know what exactly you need without some experienced support in the room, and some financial commitment. They are also very intrusive in a small space. Visually and physically.
Over the last year, we underwent what I like to call "project make it cozy" based on some feedback from my wife. She very directly said "I think I'd like to spend more time in there if it was cozier." So we got new decently heavy curtains, a new solid wood rack, new artwork, some light diffusion artwork on the front wall between the speakers. The space is much more visually inviting these days. And you know what, we spend more time in there together than we ever have, so it's been a great success!
All that said, any room treatments added need to keep it cozy. Because if it starts to feel like a sound studio, that pushers her out, and pulls me out to an extent. I'll likely revisit treatments in the future, especially if we move or add on a larger space to the house. But that's not anywhere in the foreseeable future, and treatments remain fairly low on the list for the time being.