Sorry, but your formula is for DC circuit, not AC (50Hz audio is a 50 Hz sine or square wave).Get a test tone. (50hz should work) realmofexcursion.com has test tone downloads.
Disconnect the positive lead on the amp or sub
Play test tone
Connect an ammeter in series with the sub (this will be likely more load than your DMM can handle, they are usually rated for 10 amps, my amp will output 20+ amps)- Set to AC
Measure voltage output of the amp on same test tone and volume.
This is assuming no clipping. You should do it without bass boost engaged, flat eq settings, etc.
I think that's right