6.3mm to 3.5mm adaptor question

[Soulwax]

So I've been looking at some headphones for use with my laptop, but mainly with my iPod, with a frequency range of 5-40,000Hz.

But the only headphones I can find have 6.3mm connection, will using a 3.5mm adaptor result in any loss of sound quality or frequency range?

Also, if I'm using these with an iPod full of mp3 / whatever the Apple format is, is such a frequency range even necessary? I just want to get the best sound quality possible out of it.

Thanks!