You don't need any sort of crossover, just hook it up normally. You'll want to plug the computer in as a Line-In. If your amp doesn't have a line-in, then just do it to the regular input. As for the low pass filter, you shouldn't need one, it should just run fine on it's own. If you google, "How to use a guitar amp as a computer speaker" you will find all the results are people explaining that it works as is.