What all of you guys are trying to describe is slew rate: how quickly an amplifier can rack up the voltage. An example of a top-flite headphone amplifier is AMB's B22 - it has a slew rate of 198V per micro-second. For comparison, one of the faster opamps available - the AD8397 - has a slew rate of 53V per micro-second. The classic OPA2134 (used in the CMoy) has a slew rate of 20V per micro-second. Those specs for opamps are "laboratory conditions," though. There's an awful lot of extra amplifier circuitry necessary if they are to provide that to a meaningful load. The B22, on the other hand, meets that 198V per micro-second level while providing 49V point-to-point.
Very interesting read! I also did not realize slew rate can vary so widely between amps. I wonder how fast a slew rate increase would stop making an audible difference? I am surprised the O2 is so slow. It is a good sounding amp. But with the right amp and headphones, I am sure I would notice a difference. What do you think?