If you plan to change the speaker, do read through this thread before settling on what you want to do. From the Fender Rumble Wiki: Fender Rumble V3 100 speaker up-grade ? A bit of a longer read, but worth it.
My preference would be the S2012 with the appropriate ports and cab lining. That should make these great amps even better!
I'll probably go with that speaker since a local speaker shop down the street from my work place has them