I have this little sub thing that came with some old computer speakers and it’s great. It has left and right speakers on the back and I have connected some nice big speakers and having the extra base is great. I want another speaker setup in a different place but I don’t have another sub thing. I can’t seem to find any or another way to connect a sub. All I want is some decentish bookshelf speakers on my desk (one left and one right) with a little sub under my desk for some extra base. How can I do this? Thanks