Can I put my old sound system in my Prius? please help.

I think you would be fine powerwise. The real bottleneck in 12V power for the prius is the DC to DC converter that charges the 12V battery and powers 12V accessories from the high voltage source. I can't remember the ratings of that converter right now but when I last looked I determined that it would be okay to install the Rockford T8004 amp (400-800+ Watts total) that I'm putting in right now. Bigger amps use roughly the same amount of power as smaller amps unless you turn up the volume past what the smaller amp can put out. So if there is a problem, just don't crank the volume up all the way and you'll be fine. My take is that putting a bigger amp than you need into the cargo box is good, because bigger amps have better thermal management (heatsinks etc) than smaller amps, so they will be less likely to overheat at moderate listening levels.

Also note that if you are trying to power the factory speakers with the aftermarket amp, you will need to post which audio system (JBL or not) you have in order to get more information. There are obstacles to powering the JBL audio system from an aftermarket amp.

If you have the JBL system take a look at my doc:

http://www.tom-morrow-land.com/tests/priusaudio/

 

Yep, I think you would be fine using that amp for driving the subs, and even for driving all the speakers in your non-JBL system. I was considering getting that amp for my Prius. Unless you are a bass fanatic and want to use all the power for your subwoofers, I would hook it up so that four channels drive the four factory speaker channels, and use the other two channels for the subs. There is no use in putting hundreds of watts of power into the subs when the main speakers are getting much less power. That is unless you like really boomy bass. The only downside is that it will probably cost a bit more installation money to route everything through the JL amp.

The only thing to watch out for is heat buildup... that amp doesn't have much in the way of heatsink area, so if you can mount it somewhere non-enclosed, that would be better than putting it in the cargo box like most folks do. But even in the cargo box it'll probably be fine as long as you don't crank it up all the time.

 

The DC-DC converter will put out what a typical alternator would on a conventional car. Say about 80A for safety's sake. It is fused at 100A.

The aux battery can deliver the current, it has a 150A fuse on it, but only delivers about 38AH. So a full draw won't last long.

In case you aren't familiar with the terms, AH is Amp-hours, or the amount of current in amps * the amount of time. So if you draw 76A out of a 38AH battery, it will last about 1/2 hour.
Watts are A*V, so for a rough estimate on min current draw of an amp running at full load, divide its peak watts by 12, and leave plenty of room for conversion losses.

 

You will definately want to replace the factory speakers if you are adding an amp. Those are the cheapest factory drivers I've ever seen! They won't last long trying to keep up with your bass.