It's not only about noise generation where simply looking at impedance will address, it's also about dynamics, punch, depth of presentation etc. For that you'll need good current, both in quantity and speed of delivery. An iPod isn't up to that.
The SR325is has the same impedance as the RS2i, but it really sounds bad on an iPod, i.e., flat, compressed and with metallic thin highs. OTOH, with a very well powered headphone amp, it comes to life and changes quite drastically.
The RS2i does sound better than an SR325is through an iPod, but again, it will do quite a lot better amped. It's interesting to spend US$500 on a pair of cans that sound a lot better amp'd. But to each his own.
For my iPod only, I quickly go the IEM route.