I would also consider a pair if it really made a difference for me in my room
Music is stereo, not mono. If you're new to subs, get a couple $500 subs and connect them in stereo near the speakers, especially if your speakers only go to 50Hz and you have to use a higher crossover that can be localized. You don't want bass that should be coming from the right speaker come from a sub behind you.
However, with tower speakers down to 30-40Hz, you have the advantage to have your cake and eat it too; you get stereo bass from the speakers down to 40Hz and you can place subs elsewhere for more coverage. You can also operate the subs in either mono or stereo since bass is harder to localized below 40Hz.
If you want more slam, which is above 50Hz, you will have to raise the crossover. With towers, you can keep the crossover low and still get slam. Anything below 30-40Hz is smooth and warmth, not slam.
Generally the crossover on the sub should be sufficient to minimize any overlap with the speaker. If you want more slam from the sub, use a higher crossover. But you then need a preamp with digital EQ to knock down the overlap with the speakers.