Output PowerNormally an amplifier's output power is directly proportional to the impedance of the load being driven.
E.g.- An amplifier rated at 50 watt/channel into 8 ohms will deliver 100 watts/channel into 4 ohms, unless the amplifier circuits are intentionally designed to be current limited. Some amplifier designs can drive loads down to 2 ohms or even less. With very low impedance loads, the maximum amperage rating of the output fuses of the amplifier become the limiting factor. The fuses would blow long before the maximum theoretical power level is reached.
Conversely, the same 50 watt amp would capable of delivering 25 watts into a 16 ohm load, 12 watts into 32 ohms, 6 watts into 64 ohms. and 3 watts into 128 ohms.
Some amplifiers, most McIntosh amps come to mind, use output transformers that match the output impedance of amp to the load being driven. With this design, the output power level is always the same, within the range of the different taps available at the output transformer, i.e. 4,8, or 16 ohms. The ouput of a 50 watt McIntosh amp equipped with autoformers (McIntosh's trademark name for output transformers) would be 50 watts at any of the above speaker impedances.
A 10 watt/channel amp (rated at 8 a nominal ohms), which is the minimum recommended power output for driving the K1000 Earspeakers, would be able to deliver approximately 0.625 Watts continuously at the rated impedance of 120 ohms for these headphones. This does not seem like much power, but remember that most headphones need far less than 1 Watt to achieve a 90db level of sound. 90db is plenty loud, loud enough to cause hearing damage over an extended listening period.
Also many amps can deliver, for brief fractions of a second (a few hundred milliseconds), power output levels that are 50% higher, even 100% higher (double their rated power) or more without clipping. This is a good match to the power demands of most music, where the average power levels are 10db or more below the peak level. 10db represents a 10X increase or decrease in power demand.
A seemingly modestly powered 10 watt/channel amplifier can handle the peak power demands placed on it because the average power output is rarely even 1 Watt of power, so a 10db musical peak would require no more than 10 Watts of output power from the amp to avoid clipping the signal. Factor in some amplifier output headroom of 50-100% for momentary peaks and the 10 watt amp should be adequate for producing sound levels as loud as most people would ever require, assuming either an average sized room and loudspeakers of average or better efficiency, or headphones capable of producing at least 90db sound levels with less than 1 Watt or less of input power. Also remember that amplifier clipping, while undesirable, is not usually audible during brief musical peaks.
Increasing the size of the room significantly, or selecting low efficiency speakers or headphones, while turning up the volume just a bit, and the power requirements could easily double. Remember that a 3db increase in sound level, which is audible but not a dramatic increase in volume, will double the power requirement.