You would not connect an outboard amp to any kind of in on your computer, line or otherwise. Line out maybe, which would use the computer's own dac to send an analog signal out to an outboard amp for amplification. Usually a line out from a computer is just via the headphone out with the volume turned up all the way. This will be double amplification if you are running it to an outboard amplifier, but is usually unavoidable as most computers don't have a dedicated line out. Best practice is going to be using an outboard dac and amplifier, but you'll need to get a digital signal out to the dac via either usb or s/pdf (usually optical via toslink).