I've tried a few subwoofers. Good subs are big, sealed, expensive, and start running below say 70hz. There very little piano information in those frequencies. If you have good loudspeakers, a good sub adds the slightest bit of low end and visceral sensation. Subjectively maybe a 5% improvement, at most.
If you have decent sized/quality speakers, rather than buying a sub, I would recommend tweaking the placement of your speakers and maybe some room adjustments (rugs, bookcases, curtains, etc). These free adjustments are the best audio tweaks you will find.
More comments on subs if you make it this far...
Subs take a long time to set up and adjust, including tweaking the room. Proper setup requires measurements (e.g. REW software).
Subs can be great for pipe organs, action movies and some music.
Small unsealed subs are great for 3-way computer speakers. But for serious audio, they won't play too low and will be easy to locate. If the sub plays above 70hz, you likely will be able to locate it which ruins the effect.
Hope that helps.