I agree about the output level. This is basically because the unit was made for headphones and not for line level use. Radio Shack, at one time, carried preamps. Not sure if they still do... You might also want to try pro audio/music shops that cater to bands and musicians, such as Guitar Center. They probably carry something that would fit the bill.
You can get plenty of sound level out of the cyberhome AC3 decoder without buying anything. Just use the "inline" amplifier. It is the black lump inline with the headphones. Cut off the headphones down line from the black lump and solder a RCA cable in its place. If interested let me know I'll look up and post the colors of the hair wire you need to connect. The headphones have 6 channels so you don't use all the wires.
That's the device I was thinking of. Its too bad Rat Shack phased out many of their more useful items in order to stock the shelves with Wal-Mart fare...