Bottom line is the same. If you want to drive large speakers, with lots of volume, then you need an amplifier to do it with. There are a dozen different ways to get amplifiers and to store amplifiers, but you must have one.
Your TV, for example, has a built in amplifier behind it for the tinny little speakers that come built into TVs.
But, they ALL have amplifiers!
So, for options, you can use wireless speakers, which have internal amplifiers. You can use powered speakers, which also have internal amplifiers. Or, as is the most typical solution, you can just put an amplifier at the box and use it.
The Parts Express amps listed above are a good way to go, or they have an even smaller one, about the size of a paperback book...
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=310-310
There is not really another option to this as speakers take power, and RCA outputs are not powered.