Last 2 posters, this is a work on progress as I think of where the best spot for attenuation is, try other amps, and make more progress. For instance, I need more attenuation for my system, so I have been loading up on the series resistor before the phones. This brings the amp to a higher power point, too, so I am changing 2 things: output of the amp, and impedence the headphones are seeing. However, the amp is still seeing the same impedence load. Changes in sound could be from running the amp at a watt instead of 1/4 watt, for instance. Or from running the phones from a current source, where the current will be constant in the face of changing impedance. This is neither right nor wrong, depends on the headphone, how it was developed/tested, and the listener.
In a way, part of the different sound of different amps into a particula phone would have to do with the output impedance, not quality of amp. It could be an advantage in having enough power to try the amp both ways, as a voltage source and as a current source.
Since this is not a finished work (though I love what I heard this morn, running the HD800 from a current source tames the high end and brings out the bottom, as well as richens the mids) I don't want to be misleading readers into thinking this is a finished work with definitive answers. Many forum guys make one experiment and issue dogmatic statements. It takes a long time and a lot of testing and measuring to really figure out what is going on and black and white answers are often not available. For that reason, int might be best to pm me and I'll let you know what I'm trying and how it works. I might be able to give you some direction based on the headphones you are using. No promises.......!
Have you made any more progress?
I'd like to try this with a vintage amp. Seems very strange to me that more people aren't trying this.