Enough Power

I am installing 2 Diamond Audio D9 2 channel amps. The input fuse is rated for 125Amps. One will be at 800 watts, the other at 400. Diamond's manual states they don't recommend a capacitor in line because amperage is more of a concern. They recommend a high power alternator. Some advice I got was use a single Stinger SP2150 isolated from the stock battery with an SP200. Then if the alternator eats itself down the road I can have it rewound for higher output or buy a new Stinger. Question is the SP2150 over kill. Other ideas would also be appreciated.

Diamond gave good advice.  Make sure you have an alternator that will produce the power your audio system and car system need, as you'll be pulling up to 185 additional amperes from the alternator.  You should start looking for a 225 -250 amp alt.  Keep in mind this is at full blast, but we all know that full blast is not out of the question;  we all push the boundaries as far as they will go sometimes.   Also upgrade the power/ground wiring under the hood.  Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Thanks for the advice stevdart, got the wiring upgrades covered. Is the battery still necessary with a alt the caliber you are talking about. I am not planning on running the system unless the car is running.

A second battery's only function is to provide more amperes than one battery can supply when the car is not running.  The audio system will need constant power supply, and the alternator is the car's only power supply.  It has to supply, at a minimum, all the draw that is put on it.  The battery will draw, as well, if it is starting to get depleted, so you don't want to try to run your audio with the intention of pulling from both the battery and the alternator.  The battery has first dibs on power...it's not going to wait until you turn off the audio system before it starts getting recharged.  Spend your money wisely:  upgrade the power supply and associated wiring and leave the other junk (as pretty as it may be) for the uninitiated to experiment with.Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.