Doctor FTLD said; "The headphone specs are 100db/1500mW, so this headphone need at least 1.5W amplifier, better if 4-5 Watts!! " This guy probably dives 100m to watch the time, as his watch is 100m water proof rated!!

How much power should deliver a headphone amplifier? Simply, enough to fully drive the headphones, and not too much to damage them and/or the listeners ears if misadjusted.

First of all we must read properly the headphones specs. Sensitivity and maximum power are the most important figures to determine the amp power needs.
The cans builder use two ways to indicate it, Sennheiser and other German producers use the sound pressure per Vrms. For example the HD650 deliver 103dB with a signal of 1Vrms (or 3Vpp) 1Vrms on 300 Ohms means about 3mW. Therefore with 3mW output you have on your ears the sound pressure of 103dB.

Sony and other Japanese builders make things easier, they calculate sound pressure directly in db/mW. Example the Qualia 010 is 100db/mW, this mean 100dB sound pressure  with only 1mW on 70 Ohm ( headphone impedance)
How much is 100dB? 100dB is the level of noise of a train passing at 10mt from listener,  or in a more audiophile way the maximum sound pressure in a classic music concert hall. So with 1mW and the related 100db of sound pressure you can hear a classic concert at natural sound pressure…Incredible!!

What happen if increasing the power?
Doubling the power (measured in mW) means increase sound pressure of 3dB (decibels are logarithmic, decibels related to power dB/W increase of 3 dB at power doubling) therefore with a 100dB/mW headphone, like Qualia 010, you will reach 112 db ( the level of a live rock concert or techno disco) with only 16mW, and the threshold of pain and hear damages of 121db with 125mW, the use of this headphone with more than 125 mW is impossible and very dangerous for your hearing.

Therefore an amplifier delivering 500mW on 70 Ohm ( example of RP010 with Qualia 010 ) give four times more power of really needed by headphone This amplifier fully drive the headphone at ¼ of his power.
You may say: "why the builder indicates a maximum power of 1500mW?" Simply this mean that a headphone drivers will burn at 2W, this do not mean that headphone must be driven at this limit. This is the safety limit! Since Qualia 010 is a very well built headphone the safety limit is very far from real usable limits.
A clear example of this is what declared in official Sony CD3000 specs: 104 dB/mW maximum handling power 100mW/ rated power 500mW, this mean a sound pressure of 104 db with 1mW, a real listening limit of 100mW (~123db), and a headphone safety limit of 500 mW!