are you using the a5 or a5+?
could you verify which one of these your back panel looks like?
http/images.macworld.com/images/article/2012/07/audioengine20rear-289275.jpg
since they are powered speakers you should be able to connect up via 3.5mm just fine, what seems to be the problem?
you should be connecting to the green front speaker jack on your motherboard and double check that the speakers are set as the default audio device. also make sure you do not have headphones plugged into the front of your pc at the same time.
if that is not the issue... then you need to be a bit more specific.
as far as other speakers are concerned.... connecting powered speakers up to your pc shouldnt be an issue (such as the a5). connecting unpowered speakers however would require an amp if they are anything with more than about 15w rms. i'm not sure what other speaker set (other then the a5) you were reffering to.