I will try not to be redundant. Trojantrow pretty much delivered what came to my mind.

I will contribute....

Yes, REW with a MiniDSP) microphone is a great tool. Everybody should have one. It saves SO much time helping you to nail down what you are hearing, or not hearing, to help you address it. If you're getting nulls due to standing waves, at some frequencies, what you're hearing is less than the driver is actually putting out, in which case, a change in location may be warranted, or at the minimum, maybe doing an audit on any 'correction' to perhaps override its attempt to 'correct' the room by dumping power into a frequency that can't be 'fixed'.

A basic volt meter can help you both level match, as well as determine actual signal levels, at the input of the amplifier, and the output of the amplifier. It does not have to be expensive to be good enough. If you want to get a Fluke, I wouldn't tell you not to, but most of them will measure within the subwoofer frequency range just fine.

I would suggest, though, that you start low, do it methodically, when you have no other distractions, because to measure it requires a continuous tone for several seconds, and because it is not peaking, it also means it is not NOT peaking. I.e., you can fry things, especially your subwoofer. At higher frequencies especially, you can burn out the voice coil without mechanically stressing the motor assembly, exceeding excursion, etc. Point being, you could be on your way to smoking your driver, and it sounds relatively fine until it suddenly goes quiet.

If you know your voltage, and you know the impedance at that frequency, you can determine power (or at least apparent power). If you know the phase angle of the load, you can determine real power.

Watts=VoltageXAmperage*Cos(PhaseAngle).

If you measure the voltage, and you know the impedance, you don't have to measure the current, you can calculate it with Ohm's law, E=IR (or V=AR / Voltage = Amps*Ohms), solve for Amps, Amps=Volts/Ohms, and then plug it into the power calculation to get

Watts=(Voltage^2/Ohms)*Cos(PhaseAngle)

Or for the sake of what we care about, what the amplifier is 'putting out', just don't worry about cos(phaseangle) and

Watts=V^2/Ohms

If you really want to know to be accurate, you should measure the current too, because the impedance will go up as the voice coil heats and the resistance goes up, and/or for that matter, measure the impedance plot immediately after to see what it is doing when the coil is heated to the temperature it gets to at that output level, but then again, the heat it would get when you were measuring it continuously would not be the heat it was getting if it was dynamic so...

I'm really spinning off base.

Point being, you can measure the voltage level at the input, at a certain gain setting, and see how that tracks with your entire range of operation.

For instance, if your 'system' volume is at -6 dB, and your maximum range is 0 dB (reference level), and you run a signal in REW via a digital connection of some sort at -6 dBFS (6 dB below maximum signal you can record on the source), then you are - 12dB from the maximum possible voltage output level (0 dBFS, 0 dB system gain).

Note, I am assuming no EQ or filters involved, so whatever they do is compounded. That is pretty close (but not exactly) 1/16th power level, and 1/4 of the voltage level of maximum output, and it is easy to multiply by 4.

So, on the input of the amplifier, you want to know your input sensitivity of the amplifier (signal level in volts that drives the amplifier to full output). If it has gain adjustments, that level is... adjustable, in which case you would like to run it so that the output of your preamplifier is near (but not beyond) it's maximum output. Keep in mind, though, that if you've got EQ involved, this level will measured will change with frequency, so either measure it at the frequency of maximum boost, or run it low by whatever amount that maximum boost it. But this is really a matter of signal to noise ratio, so long as the signal can get high enough.

On the amplifier output, you don't really set that, but if you measure the voltage at one gain setting, since amplifiers are pretty linear (or they should be), you know the output level well enough at all levels until you exceed their output capabilities. So, if you measure the amplifier voltage and it is swinging 10 volts (which should be safe to run continuously into that driver, and you are calculating estimated power at a 4 ohm 'nominal' load, which is good enough for us, probably,

Watts=V^2/Ohms

Watts=10^2/4=100/4=25 watts * 4 ohms, 12.5 watts at 8 ohms, 50 watts @ 2 ohms, etc.

Every 6 dB of gain adjustment is about twice the voltage, so if you wanted to run a 6 dB table from there (roughly)

Voltage | Ohms | Power |

5 | 4 | 6.25 |

10 | 4 | 25 |

20 | 4 | 100 |

40 | 4 | 400 |

80 | 4 | 1600 |

Hopefully that's useful, and not just a plain mess. If I screwed anything up, you can have a refund of the consulting fee