I use the Yamaha HS8S sub with the HS8 monitors. Along with guitar, I play bass, synths, edrums, and of course, recorded music through them. To me, just using the monitors without the sub is like using headphones. You can hear the bass, but you can't feel it, especially at lower volumes.
I set mine up to be just perceivable, much more than that and it can be overbearing to me.
One thing I found was that using a sub really makes low cuts necessary on guitar patches. They can get really dark and thumpy otherwise.
A pair of subwoofers? I can't see needing this unless you're going to be playing really loud.