Small Speakers/Small Room.
It's not just about ADDING more bass, it is also about letting your 'small' amp and 'small' speakers do a cleaner job, no trying to do what they really cannot do.
The best thing is to take the low bass load away from BOTH your amp and away from your small main L & R speakers. Then both the amp and the speakers do not try to make low bass, lets both do a great job. Get the signal to the sub at line level, back to the amp after the sub's crossover.
Self powered sub to add just enough low end that the system does not come off as 'small'. Not really aware of the sub until you turn it off.
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i.e. my office, using a small tube amp 10 wpc.
Prior setup was a pair of wharfedale diamond 225's (6.5" driver, sensitivity only 87 db) and a self powered sub.
next, I restored a pair of AR2ax's with 10" woofers. I was going to use them in the garage work area. Surprisingly they are only 11-1/4" deep, fit in my office bookcase, no sub required.
Wharefdale and sub went together downstairs.
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Moving the low bass load from your amp to a self-powered sub lets you use an amp with far less power, i.e. I am getting away with 10 wpc. Importantly, that makes it far easier to try tube amps. Smaller, less money, less heat, more placement options.
Note: if you use speaker wires to/fm the sub, your amp is still needlessly making low bass, only the speakers get a break from the sub's crossover.