As a simple example, if I wanted to attach a CD Player, ipod or computer via a 3.5 stereo jack, do I need to purchase a preamp also? If so why?
No. Both the 350 and 450 can have their input impedance switched between line and mic values. You'd just set the speaker to line impedance and you're okay to connect a CD player or an iPod directly to the speaker.
If you're connecting an iPod: start with the volume at zero on the iPod and move it up slowly, watching the clip light on the speaker to make certain you're not clipping the input on the speaker with a too-hot output signal from the iPod.
I would like to point out that both models do not include 3.5mm input jacks on them. The only connection method available on these speakers is via a balanced signal on an XLR connection. You'll need an adaptor to connect consumer-grade gear to these speakers.