Thanks for both of your advice.
What I meant about the op amps being designed around a specific load impedance, can be seen on these two links:
The first in the typical "headphone amplifier" design. You can see that it references some THD measurements at 8 ohms and 32 ohms, as well as output power at 8 ohms. I'm afraid to use this op amp to drive my 250 ohm cans, as it doesn't list any figures on what the output power is when driving 250 ohms.
The second link appears to be an op amp designed for professional audio equipment. On the first page it gives similar references to the first op amp, but with load impedances (RL) of 600 ohms and 2k ohms. I'm afraid to use this op amp, as, in an electrical sense, driving an op amp with a lower than expected load impedance can be similar to shorting the op amp itself, and this could cause overheating due to too much current, and potentially damaging the op amp.
Both of these op amps can be found from National Semiconductor's website using their cool selection tool. The first one is at:
And the second one is at:
(or by clicking on the first link, and selecting "High Performance Audio")