I'm looking for headphones for a spider ii 15 watt amp. 8 inch cone. I was told that it is suggested to use 160 to 600 ohm headphones. Is this required? There are a lot of headphones out there, but most of them seem to be under 40 ohms. What will the difference be, and do lower ones work at all? Can I just buy a 1/4" adapter and use regular over the ear headphones?