I am using a pair of bose computer speakers for my computer that have no sub

I also have an old fender bass cab with a 15 inch eminence speaker that I mainly use as a cart for my guitar interface, mics, rokits,etc It's just a speaker no amp. I was wondering if there is a little amp I could plug to my computer with some kind of crossover with maybe like a lowpass switch and run it to the cab to give my computer a little more thump.I just thought it would be convenient because its sitting right there already

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.