Most amps can be attached to rca or with speaker wires on terminals. Most stereos with rca outputs have only low level output on those cables ( like for a subwoofer ).
Easiest way to read up on all this is download the manuals for some aftermarket stereo and/or amplifiers.
Please understand that most stereos are 4 ohm per channel or 8 ohm per channel.
2 speakers at 4 ohms on the same channel equals 8 ohm load. 8 ohm load on a 4 ohm channel wont last very long.