[satan]

Amp on the pre-'06 GL1800 is the radio itself (not an external unit).

A few aftermarket people supported modifications to the radio (updated output amps) or the addition of an external aftermarket amplifier (Electrical Connection had/may-have).

 

[satan]

Yep -- That's a good internal mod for the early radios (as alluded to above) -- I've heard a few of em and they sound great!