Depends a bit on the impedance. High impedance (>100 ohm) headphones will be quieter - eg some really high end ones are 600 ohm and require a lot more voltage swing to drive them well.

The Karma has about a 3v p-p swing on the output. I use it with Sennheiser 691's without a problem, loud enough for me... I think these are 64 ohm.

Hugo