Adding an amp: best way to convert high level speaker outs?

Is it better to use an external "step down" to convert the 04/05 Prius head unit's high level speaker output signals down for use with an added amp's RCA low-level inputs or will a high quality amp handle the conversion internally as well or better?

In either case, how much extra distortion should I expect (compared to if the Prius head unit had RCA low-level outputs to begin with) and will using a "Class A" amplifier help because those supposedly have less distortion?