Ok, well that rules out the internet direct companies as well then.
You will notice a big improvement over the logitech speakers. Logitech speakers use very small speakers and therefore cannot move a lot of air. It really is not a good idea to runs some small bookshelf speakers as full ranger as they simply cannot reproduce those low frequencies. Especially those that are below 80hz. If you add a sub pretty soon you will have a system that should last a long time.
What the real killer in speakers are is distortion which you will hear when you start to run into problems. When you hit this point, turn down the volume and your speakers should last a long time if you dont put your finger through the surrounds, etc.