So.. I just listened to my H-100 with a 75 ohm adapter. My iPhone 4 can still drive it effortlessly (though the volume needs to be bumped up more due to the increased resistance).
I enjoyed the sound of the H-100 without the adapter, but results with the adapter are very positive. It might resolve a lot of issues for those who were less than enthused with the sound.. namely, the 'thin' midrange & upper midrange/lower treble peak. I might even like the sound with the adapter even more than the 'stock' H-100. More impressions coming later.
This is very interesting! Any thoughts on how adding even more impedance (above the 75 Ohm you tried) will affect the sound?