johnstaf There's very little low bass in a piano, so I wouldn't bother with a subwoofer. When playing even the lowest note on a concert grand you mostly hear harmonics.
I have not paired a decent subwoofer with a digital speaker system, so take this for what it's worth--but I can see why it would seem desirable. On a large grand, when struck moderately loudly, there's a huge reverberation that you don't necessarily hear, but you feel it cause your body/head/chest to vibrate. Even if I cough or sneeze at the bench, I can hear and feel the piano's soundboard respond and it in turn causes a physical rumble feedback that I can feel in my body.
I've never gotten that feeling with a stock digital sound system (even augmented with 5" drivers), but the cough thing aside, I can image why people might seek a subwoofer for that more visceral response that is lacking with regular speakers?