I have a Creative ZXR sound card which I've been using with a set of Altec Lansing ADA995 5.1 PC speakers. I've recently been offered a set of Monitor Audio V10 x 4 plus a Monitor Audio V20 center speaker. Would this be a worth while upgrade to my current set up replacing my existing satallites? Could I add a different sub to the system at a later date without having to buy an AV amp?