Micro Terror headphone question

[suncrush]

In the interest of getting to keep my wife and my new guitar, I'm getting some use out of the headphone jack. I'm using a 1/4" to 1/8" adapter and a pair of Sony earbuds right now, which suck.

I'm looking at getting some Sennheiser cans, but I've had really uneven performance with borrowed headphones. I tried two sets of Beats by Dre, both of which were working normally plugged into an iPhone. With the MT, one set had virtually no power, and got really crackly with the amp cranked. The others worked well.

Could this be an impedance issue? If so, what impedance is the jack meant to work with?

Any other guess what's causing the issue?

Thanks a heap,
Brian

Also, love the amp and it's great to be part of the community.