Had to put in my thoughts in this thread.
I've been through quite a few computer speaker systems myself - a lot of Altec systems, because time and time again, they don't dissapoint.
Altecs - ACS33, ACS44, ACS45.2, ATP3, ADA880, ADA885, ACS54, ACS56
and the Logitech Z560's.
I currently own the ADA885's, ACS56, and the Z560's.
The Z560's are the system to get if you want to fill a large room with a ton of sound (and bass).
It seems that logitech just took the brute force method of creating LOUD speakers.
They do sound good at loud volumes, not much distortion in the bass at all.
However, the details in the music are not as bright as they are in the ADA885's.
I prefer the ADA885's for music and movies, and the 560's for gaming, and songs with lots of bass.