What kind of "line" are you working with? Some are balanced, but the
circuit you refer to is for an un-balanced line. For a balanced line
you may need an audio transformer.

the line input is a 1/4" TRS socket on an m-audio microtrack II.
http://www.m-audio.com/images/global...flyerUK_LR.pdf
Strangely enough I haven't been able to find any proper specs on the
microtrack but I know it commonly takes microphones and such. Headphone
output is too 'heavy' for a decent recording into this input so I need to
attenuate it.
I can't use any boxes or such because the headphone device and the
microtrack will be used on the move, often in the open, on foot, running on
batteries. Thus I have to integrate any attenuation into the cable. Thanks
for any further advice.