The K1000s are extremely inefficient. They are rated by the manufacturer to produce about ~100dB at 400 milliwatts at 120 ohms.
I use my K1000s with an amp rated at 10 watt per channel at 8 ohms. If I've done my Ohm's Law correctly, that comes out to only 666 milliwatts at 120 ohms. So I would need the output volume of this amp at about 2/3 its max to get to 100dB sound pressure, although that's way louder than I would normally listen.
In practice, for most of my listening, the volume control is anywhere from 30 to 50% of max, with a 10wpc amp. If I hook up the same amp to my loudspeakers (8ohms, rated 89/dB per watt) in the same room, at the same volume setting, the speakers are noticeably louder than the headphones, despite the headphones being right on my ears while the speakers are about 10 feet away.
It must have something to do with the unusual way the diaphragm and magnet structure works in the K1000s, and the fact that half the sound pressure goes out the back side of the wide open earpieces.
This week's favorite CD: The Very Best of John Lee Hooker