Yes it can be done.
The question is why would you want to do it.
The facory HU puts out 1-2V, that's horrible.
I could introduce distorion into that with a hair dryer or a well placed magnet.
A bad aftermarket HU puts out 2-3V, a good one puts out 3-5V, and a great one put out 5-8V.
A competition sound system will work between 12V and 14V.
How much voltage you HU puts out is directly related to how loud and how good your sound system will sound.
The average street person runs around 5V.
There is no point in amplifying a signal (regardless how could of an amp, if the signal is crap).
1-2V output effectively decreases the range of frequency your system can reproduce.
Why? Because it take more power to reproduce certain frequencies.
The average human hearing range is 20Hz to 20kHz.
A competition sound system will reproduce one octave below and one third (1/3) octave above that. That makes the range 10Hz-25kHz, which can be done at 12-14V.
5-8V will do from 35Hz-20kHz,
3-5V will do 40Hz-18kHz,
2-3V will do 55Hz-15kHz,
and 1-2V will do 80Hz-12kHz.
