@Norman, The LCD2 is a planar magnetic (also known as orthodynamic) design, so the way the driver works is different from conventional diaphragms, hence the resistance is flat. This has advantages in terms of accuracy, but also costly. Regarding the amp - headphone matching, most music can hit peaks of 110-115 dB, for very very short bursts, based on the genre, but its not to be confused with the average level, 70-80 will be pretty loud for most ppl. It is this peak that the amp needs to sustain. So given the sensitivity in dB SPL /mW, the power requirement can be found from the formula I mention in my thread. Now consider an amp to be a voltage source. Normally you'll find two combinations of headphone specs: High Impedance, Low to Moderate Sensitivity: These are tough to drive, and will need high Vrms to get to the same peak levels. Hence an amp is recommended. Common example is the HD650, HiFiman etc. Low Impedance, High Sensitivity: These are meant for portable use, they'll need lower Vrms to get going, but will consume higher current, a characteristic of how batteries behave. Won't work well with amps that have high output impedance. Generally a factor of headphon impedance being 8-10 times (or more) the amp output impedance is considered sufficient. Regarding the part about keeping the volume levels at 55 dB, the thing is you won't be able to increase it by much in case of portable devices, because they'll max out in terms of the voltage, and you'll find certain frequencies to be numbed down (not the case with orthodynamic) because of the frequency-impedance variability.