You weren't seeing 190 watts, you were seeing 190(VA) volt amps. The power formula is different for AC signals than DC and speaker outputs are AC. The power formula you used is a DC formula. Gain should be set and not messed with, as it is not a volume control for the amp. If you've have the gain set at 3/4 as you mentioned above, you were almost assuredly clipping the sub(s) and, that's likely the reason you damaged a previous sub.
With a properly designed and built ported enclosure, those subs will handle a 1000 watts of clean power and they will be remarkably louder and will have much better low end response than they do in a sealed enclosure.
As for your voltage drop, you're on the edge with the alternator but your hands are not tied. You can add a second battery.