Which pair of Ultrasone cans would the most easily driven by an Ipod Touch?

 Which pair of Ultrasone cans would the most easily driven by an Ipod Touch?

The

1. Ultrasone HFI-450,

Frequency range 20-20.000 Hz

Impedance 32 Ohm

Sound pressure level 96 dB

2. ULTRASONE HFI 580,     

Frequency range 10-22.000 Hz

Impedance 32 Ohm

Sound pressure level 101 dB

Or the,

3. Ultrasone PRO 550?

Frequency range 10-22.000 Hz

Impedance 64 Ohm

Sound pressure level 102 dB

I'm just looking for a powerful sounding pair of over the ear headphones with detail and powerful yet well defined bass for driving with an Ipod and no headphone amplifier. Which one or ones of these 3 Ultrasone cans would do this best?

-Thanks

Also I understand that a 32 ohm impedance is better for driving an Ipod correct? How does a 64 impedance hold up?