Sorry, I forgot to check back for quite some time, I guess!
So, much in the way that the Sierra-1 Towers were created; a bigger driver for bass extension and more midrange capability (if I'm recalling correctly?). For instance, I do not have a subwoofer right now, but perhaps you mean that combining a sub with a full set of Sierra-2 monitors would make a tower a pointless endeavor? The bass extension on the Sierra-2 monitors is quite nice on their own, but I know there is further detail there I do not hear without a subwoofer; and then there is the level I cannot even hear, but can be "felt."