James,
The 600 ohm load specification guarantees that the output performance will meet specifications. However, it will not harm the 4461 to connect low-impedance loads such as headphones directly to the outputs.
Three things will happen when you connect a low-impedance load to the output:
1) The signal level will be reduced because there's already a 22 ohm output impedance between the output terminals. The 22 ohm impedance and your headphone load will form a voltage divider that will reduce the voltage at the headphones.
2) The 4461 output stage is current-limited to about 25 mA, which will cause clipping if your voltage is at least moderate. If your headphones are, say, 40 ohms, you'll only be able to deliver 1 V into them before the waveform clips. Of course, the actual impedance of your headphones will be a complex function of frequency, so it's hard to say exactly how much voltage you can really put into them, but you won't get more than about 25 mA of current.
3) The distortion will increase even before you go into clipping (though it's likely you wouldn't hear it unless you have golden ears and magic headphones).
If you can live with those impairments, then you should have no problem driving phones directly with the 4461.
Good luck,
Ed L.
