Rather than jumping in with a second battery and all that, which is probably not completely necessary here, I would recommend at a minimum 4 ga. power & ground wire for the amplifier.
Then upgrade/supplement the wiring from the alternator to the battery, the battery to the chassis ground, the chassis to engine block ground all with at least 4ga wiring.
Let's remember that music is dynamic and the amplifier is not pulling a full amount of power continuously. Because of this, one can get away with doing as I suggested above so long as the alternator and battery are in good order.
Also upgrading the wiring as I suggested helps prevent voltage decreases which are the big enemy in car audio. The lower the voltage drops the higher the amperage demands become.
I would recommend at least 2 million micro-farads of capacitance for a 1kw rms amplifier along with the upgrades I suggested. All of these things together should allow the system to operate at high performance levels and no issues for music listening in most vehicles. Bumping up to the largest cold cranking amp battery that your car can hold is also recommended.
As for subs, dual 4ohm subs with the coils wired in parallel, and then the two subs wired in parallel will achieve a 1 ohm load. Four 4 ohm subs in parallel will also achieve 1 ohm, as would dual 2 ohm subs.
