Bass in small rooms is tricky. You physically cannot fit enough treatment in one to properly control low end (at least from a mastering perspective; I've seen the same theory applied to mixing rooms as well). For what most people consider a small home studio room, you'd be filling something like 60% of the total volume of the room with bass trapping to get an even low end response.
Personally, after having tried it, I wouldn't setup a small room with fewer than 2 subs. I'm looking forward to trying 4 in my room at some point.
To be fair...I have a lot invested in treatments, and I did that first. And, GiK consulted on building the room itself. But, finally deciding to trust a combination of people on reddit and a handful of articles (like
this one and a few that get more in depth into physics and math - look up schroeder frequency), I bought a couple and started playing with them and I won't go back.
They actually do excite room modes differently and end up just working better. Don't get me wrong, it can get loud in here if I turn it up, but at my preferred level, it just sounds right in a way that I haven't really heard before outside of 6-figure mains in even more expensive rooms.
The problems with the way most people do subs are that they only use one (there is no correct place to put a single subwoofer in most rooms) and a lot of budget-friendly subs don't let you set them up correctly. Most of the time, the crossovers that come with them are junk either in terms of quality or how they can be tweaked.
The absolute cheapest I'd consider is the LSR310S, just because you can completely disable its internal crossover and do it in software....which just works better IME until you're talking Trinnov money. That does require more outputs and more time tweaking, but IMHO, it's absolutely worth it.