> How much power (and voltage) is actually needed for the different headphone types? I guess the range is 16-600 ohms.
Nobody ever really asks that question. Try these:
http://headwize.com/ubb/showpage.php?fnum=3&tid=2432
http://headwize.com/ubb/showpage.php?fnum=3&tid=2521
The real question, as in loudspeakers, is: how loud, how clean? I was making a nice noise in my office with a sub-Watt amp and an RCA duocone speaker, often running under 1/10th Watt. OTOH I've been unable to get loud enough, clean, with 200 Watt amps. I've made 200 Watts of racket with 75 Watt amps. The headphone power I need to listen to the TV when the house is asleep is way-way less than the power I need to monitor live recording in the same room with the band. Headphones vary a lot. Impedance issues confuse things. So 5mW may be plenty, or 500mW may be not excessive.
> couldn't just about any power amp be adapted for headphones?
Sure. 50-Watt 8Ω amps are common garage-sale finds. Most of then are common-ground output, so can be wired to a standard common-gound headphone jack. If it makes 50 Watts in 8Ω in 600Ω it will make 50W*8Ω/600Ω= 0.66W or 660mW , in 100Ω you get 4 Watts or 4,000mW, in 32Ω you get over 12 Watts or 12,000mW. Since headphones need more than a mW but almost never a whole Watt, you are in some danger of frying the phones. And since you listen to speakers from >3 feet away, but phones are right on your ear, and the actual efficiency is not so very different, noise that is inaudible on loudspeakers may be annoying in headphones.
One old technique is to add a 270Ω series resistor, but this can give very large power in higher-Z phones, and I have seen a 30 Watt amp fry Koss Pro4s this way (not while on my head!). It may also under-damp low-Z phones, though I have not yet found the low-Z phone that seemed to need damping.
In the threads above I derive a 7V 29Ω output spec that seems to give "ample" power into most commercial headphones. The speaker amp makes around 20V, so we need a voltage divider with loss of about 7/20 = 1/3 and output Z near 29Ω. 100Ω and 47Ω is in the ballpark.
This also cuts noise somewhat.
The 8Ω-rated amp will see from 147Ω (no phone) to 100Ω (shorted phone). 100/8= 12 sets of resistors can drive 12 ears to ample level with total immunity to shorts. If the amp doesn't die at 4Ω you can double this. (The 100Ω resistor must be 1/2 Watt or more to resist smoking under gross abuse.)
If the amp is rated 8 Watts in 8Ω, just put a 27Ω resistor between it and each headphone, maximum of 3 to 6 shorted outputs or 6 to 12 un-shorted 32Ω phones. If rated 200 Watts, use 180Ω and 32Ω divider resistors, you can hang dozens of these on the amp.