There's no reason you can't use your speaker system with an iMac.
Granted, if you have 5.1 surround sound, you might need to purchase
a different interface or an adapter, but the speaker system itself will work fine.
My daughter just got the new Logitech USB headset and is now having a blast
recording basic vocals in GarageBand, which means she can also audio chat
or create her own podcasts with ease.