it's denpended, if you have HU at 2V and your amp is new at 4v or higher, using line driver will help you no need to change the HU (technically you don't have to) and eliminate the noise possible come from the audio source. There are many good brand out there, go to ebay to check it out with your budget. Audio Control, Alpine, Eclipse. etc
not a good idea in my opinion. will not only liftthe voltage, but the noise floor also. if u have 2 v pre outs, ya just have to turn the gain up on the amp.