You need to stop thinking watts and start thinking SPL (Sound Pressure Level). Thinking in watts is like asking how big your gas tank needs to be to go 400 miles. You need to know the MPG.
So, here's how it's done. Look up the Senstivity of speaker cabinets you are considering, it will be in dB at 1 watt at one meter. Basically, this means how loud the sound is one meter from the cab with a one watt signal (sine wave at 1000 Hz, is what the standard is I think).
Then, you determine what the power output of the amplifiers you are considering are at the impedance of the cabinet. (Solid state amps might list something like: "1000 watts @ 2,7 ohms (3x8 ohms), 750 watts @ 4 ohms, 400 watts @ 8 ohms" (Taken from Glockenklang Heart Rock II).
To get the total SPL you need to take the Sensitivity and add 3 dB every time you double the power, from one watt up to the power output into the cab. So if you were looking at an 8 Ohm Glockenklang Quattro its Sensitivity is 103dB/w/m. The Glock head above wil produce 400 watts into the 8 ohm load. Now to get from 1 to 400 you have to double the wattage 8.65 times. So the total SPL would be = 103 + 3*8.65 = 128.95 dB (at one meter). That's pretty loud.
Now if you were to pair that head with a 2.67 ohm Glockenklang 6-Box with a sensitivity of 104.5 dB, then your head would be capable of delivering 1000 watts. To get from 1 watt to 1000 watts you have to double the wattage 10 times. So the total SPL would be = 104.5 + 3*10 = 134.5. This is quite a bit louder, in fact, it's technically known as F-ing loud.
If you're trying to figure out how loud something needs to be, you should be thinking of a loudness metric. Not watts, a power metric.
Now it's true that the SPL and wattage ratings are not 100% reliable, because they are measured differently from manufacturer to manufacturer. But it is the best you're going to do.
FWIW,
hope this helps,
Jim
