I have a 450 watt head and two 400 watt cabs.
How many watts is my rig?

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Fender Jazz Bass USA 1982
Fender Telecaster Mexico 2007
Fender Precision Bass Classic 50's 2008
Richmond Fretless Prototype
Yamaha RBX4A2 2007
TC Electronic RH450 Bass Head, x2 TC Electronic RS210.

It will be 450watts thereabouts root mean square (RMS) it will reach somewhere near 800 watts peak power. I guess its a HM head.
What really needs to be measured for volume is sound pressure levels in decibels. Wattage doesnt really cover perceived volume too well. A 50watt valve amp will leave a 100watt transistor amp standing because the signal and therefore soundpressure is kept to a more constant level. The same is true of eq settings on a smaller scale. If you take 2 of the same amps set side by side one with a mid scoop the other with the mids maxed. The amp with the middle up will seem much louder as thats the frequencies human ears are most attuned to.

No doubt more knowledgable people will correct and fill out that bare bones explanation of mine.
http://en.wikipedia.org/wiki/Watt

decibels offer a better description of volume

http://en.wikipedia.org/wiki/Decibels

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No no and no

One reason for that is because not all speakers are created equally. Some speakers are more efficient than others and will be louder with the same signal being pumped into them. That speaker spec is called the Sensitivity; measured in decibels at a 1 watt input at some fixed distance.

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Member #26797
My other guitar is a Strat.

which would go some way to explaining why a marshall with a 4ohm 12" speaker will kick out more perceived volume when near the speaker than a 4x12 at 8ohms does.

Infact i think if you look on the back of modern marshall heads it says 100watt @ 4ohm / 75watt @ 8/16ohm.

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No no and no

Ohms are a measure of resistance. Power goes in - goes through the resistor, and comes out smaller.

If it goes through 4 kΩ, the signal is reduced less than if it went through 8kΩ.

Uh, nope.

Basically 450 watts. The amplifier head is the source of the power whereas the speakers/cabinets transform the electrical power produced by the head into sound power.

The fact that you have two 400 watt cabinets means that the speakers are capable of handling 800 watts, assuming that the impedances are correctly matched to the head.

Since the amp head is 450 watts, your rig is a 450 watt rig. If your speakers were only rated for say 150 watts each (300 watts total) then you would have a 300 watt rig because you would be overloading the speakers if you tried to put out more than 300 watts power, even though your amp head is capable of it.

Its more complicated than this of course but this is the most simplistic way of looking at it.

Gadabout, doesnt the peak/rms thing happen with guitar amps?

Though i strongly suspect your correct about speaker ratings limiting power.

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No no and no

I think you're mismatching terms here. Ohms can be a measure of either straight DC resistance or AC impedance. In speakers, the speaker coil generates a specific impedance to an alternating current introduced to it which is a function of the inductance of the coil:

Z = 2π fL
Impedance = 2 * PI * Frequency * Inductance

The lower the impedance, the more wattage a speaker can handle. So, the 4Ώ speaker will sound louder than an 8Ώ speaker with the same wattage applied.

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Calling an illegal alien an undocumented immigrant is like calling a drug dealer an undocumented pharmicist.

One main thing you have to remember is the sensitivty rating of the speakers being used effect how loud they will get, the lower the rating the more energy required to reach higher volumes.

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RMS and Peak are two different ways of describing power. The RMS measurement is a way of defining power that averages out the fluctuations. For the most part, the parts of an amp that have to handle large amounts of power are stressed by average power, which is determined from RMS voltage and RMS current. (RMS power is really a misnomer, but never mind that.) If you put a 440 Hz audio signal into an amp whose speaker is rated for say, 50 watts, then the average power at the speaker terminals is 50 watts when fully loaded. The actual instantaneous power would be fluctuating between 0 watts and 100 watts 440 times a second, but these fluctuations are too fast to be of any consequence. The heat generated depends on the average power not the instantaneous power.

In days of old, some amp manufacturers liked to play games when advertising the power rating of an amp. (Not Fender though.) In the 50 watt case I described above, some manufacturers would have advertised that as a 100 watt amp since the instantaneous power at the signal peaks is 100 watts, but that is just trickery. By the same reasoning, a 50 watt light bulb could be said to be delivering 100 watts at the peaks of the 60 Hz AC power driving it. But the effective light produced by the bulb relates to the 50 watts of average power not the 100 watts of instantaneous power.

Dude you have a 450 watt rig (at max output) your speaker rating has nothing to do with how many watts your head is capable of putting out.

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"Life is like eating jalapenos. What you do today may burn your arse tomorrow"

Solid state or tube?