Hi FunkyBoss
1st of all:
The 1600W that your amp is rated is only at peak out put. If you would blast your Stereo for 2 hours at peak output you would probabely be deaf... He he...
2nd:
If you get underdrive pulleys, most kits also include a smaller alternator pulley, which compensates for the slower belt speed. In fact that means that you get almost the stock amperage out of the alternator.
If that would not be enough to power all the electronics, you can still get an even smaller alternator pulley (please check availability first) and even have more output WITH underdrive pulleys.
If you would run into a problem regarding availability, a local machine shop can definitely reduce the diameter of a billet alternator pulley. (only the hollow style can not be turned down)
Have fun with your mods!
(Crank pulley is a bi+*" to get on and off!)