Virtually all bass cabinets are full-range (the more proper term). This just means they are designed to cover the full frequency spectrum of the electric bass.
Other cabs like subs, mid cabs, and separate top cabs are designed to cover only a portion of the spectrum; these are driven from separate amps. (Bi-amping or tri-amping.) This is a common situation for large PA systems, but rare for instrument amps.
Skeeter, you're asking your question about wattage, when in fact you want
volume. Nobody cares if you get your sound with 10 or 1000 watts; you just need to be heard.
That said, there are a few variables. 1. We don't know how loud you need to be. We'll assume you want to be as loud as possible, and you'll just turn it down for smaller gigs. 2. If you just build a box and slap in speakers without proper design, it will be a crapshoot. It may work well; it may not be very loud; the tone may suck; or you can even destroy the speakers.
Search out speakers that are known for their loudness. Then listen for tone.
If you really want to design your own cabinet, then read up on speaker design, run the simulation programs, and follow their recommendations. The design software will let you "try out" different speakers. It will tell you what cab size to build, how to tune the ports, and how loud it can get before the speakers fart out.
An alternative, as many of us have done, is to look into Bill's cabinets at
http://www.billfitzmaurice.com . Bill has done the design work and made the plans available.
