As a new bassist with a history as a guitarist, you'll soon find there are a number of "truisms" and points of "common knowledge" about sound reproduction that no longer apply as they once did - or even at all.
For example, the only sense in which "underpower(ing) the cab" has any meaning would be relative to the maximum capacity of the system. And since bassists generally don't rely that much on speaker distortion or power amp overloading in order to get their sound(s), this factor is pretty moot.
Likewise, it's becoming more widely known that mixing cabs of different driver sizes is a sonic crapshoot - with unforeseen issues such as phase cancellation and "hot spots" in the frequency range often resulting. For this reason, more players are becoming converted to the idea of using matched/identical drivers in their rigs, in order to provide a more consistent overall frequency response.
One issue you haven't mentioned is horizontal vs. vertical speaker configuration. Bassists (at least the better-informed ones here on TalkBass) are beginning to realize the superior off-axis dispersion benefits of using rigs with drivers aligned in a vertical, virtual line array - vis-a-vis the standard 2x12 horizontal guitar cab or 4x12 square guitar cab.
All of these departures from standard guitar operating procedure are in some way related to the unique physics of low-frequency sound, and the specialized musical functions and requirements of the bass in general.
You have much to learn, Grasshopper...
MM
