I run an Omega Claro II and a Z-5500 and I'm quite happy.
Oh, and the bass on the S550 will be more than enough I imagine (unless you're some rap/hip-hop junkie)
Definitely get the S550, the extra speakers make a nice different and will support your bass with extra midrange.
Also, something to look into would be your media sources. If your songs are 128kbps, anything you plug them into is gonna sound like crap. Make sure you have quality 320kbps mp3's or flac files.