Should I pick a 32 or 42 Ohm headphone for use with my iPad, or can I go with a 80 Ohm?
I'm guessing 250 Ohm is out of the question because of the little amp that is inside the iPad, or ?
What headphones do you have in mind? The impedance is not the only factor, sensitivity is equally important.
I'm looking at Beyerdynamic DT 250, and perhaps also DT 770.
The 80Ohm versions should be loud enough unless you listen very loudly or to very dynamic music. Don't get the higher impedance version unless your sure you can drive them.