#1
My rumble head has a headphone slot which is minimum 32 ohms. I have no idea what my headphones are (they're cheap ones) but would the majority of headphones be suitable and not damage the amp? Cheers
#2
I'm going to be blunt and say that I highly doubt you'll have a problem with whatever headphones you use. I would hazard to guess that most headphones would be suitable to use.
Gear:

ESP EC-50
ESP FB-204

MXR '78 Custom Badass Distortion
Dunlop Crybaby
MXR EVH Phase 90
MXR Analog Chorus

"Music is the strongest form of magic." - Marilyn Manson
#4
I never even thought they would bother telling you the impedance for the headphone output. Shouldn't really matter, unless you're dealing with some super high-output, amplified headphone system. I've used everything from ipod earbuds to AKG studio monitors with the headphone output of my Rumble amp. No problems. Just make sure that it is in fact a "headphone" jack, and not a "line out" or something else.
#5
Most headphones that I have used (including my current KRK 6400 headphones) are all 32ohm or more anyway. I actually think that most headphones/buds are 32ohm standard now.
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