Which senns did you go with? I owned the HD650 for a short time, but didn't like that the bass couldn't keep up with metal or other fast hard hitting music. But I did use it with a tube amp at the time so that could be a factor why it couldn't keep up. Now that I am using a solid state amp, maybe it would fair better.
I had the HD700 for a while, they were definitely much quicker (closer the HD800) in responsiveness, but the sharp sting from the upper spikes were painful to my ears over an extended period. I went back to the HD650, but that is because it works fine with my preference of music.
I thought the 'quickness" mostly is determined by the impedance match between cans and amp.