Ohm's Law when using power supply

If you want to use the ratings you listed, you must first find the power per volt because your power supply is less voltage than the test voltage.  If you go by the amp's rating at 300 watts measured at 14.4 volts, that equates to 20.83 watts per volt.  20.83 X 11.2 v, the power unit's supply, equals 233.33 watts.

If you instead use the birth sheet rating, do the same thing with 384 instead of 300 watts.  You can probably assume it's the same measurement at 14.4 volts.  The output using the power supply would be 298.66 watts.

These both assume that the ratings you posted were those for a 4 ohm load, which corresponds with your sub's impedance.

Now, use Ohm's Law to find out what voltage output you are seeking.  Let's use the power output of 298.66 watts per the birth sheet.  Here is an Ohm's Law table (from BCAE1.com):

"If you know two values you can find a third value using Ohm's Law."  What values do you know (or assume to know) in your scenario?  Do you know amperage?  Power output?  Impedance, or resistance?  You know power output (298.66 watts) and you know impedance (4 ohms).  You don't know amperage, or current.  Look at the formula on the chart that uses those two known values to find voltage:  E = SQR (P X R).  P X R is 298.66 X 4, or 1194.64.  The square root of 1194.64 is 34.56.  That is the voltage that you will seek by touching your DMM leads to the speaker output terminals that the sub is connected to (but disconnect the sub first).

But...you have made a lot of assumptions to get to that point.  You had to assume that the power supply truly puts out 11.2 volts to the amp, and  you had to assume that the birth sheet was correct in the 384 watts at 4 ohm assertion.  But if you've used test tones and your good hearing, you would have found the clipping point without the need for these calculations and meter measurements.  And you didn't mention the head unit involved here, but you know that its output has to be checked for the highest level just short of clipping before either type of gain setting is done.  Test tones are definitely needed to find that out, and they are needed for a stable reading in your voltage output test.  I have a response in this thread that will get you quickly to test tones and gain setting procedures.