These attenuators are used at the RCA ins to attenuate the input signal of the amp. It seems to be a simple voltage divider with 2 resistors per channel (GBP 40 for that, really?), so it does influence impedances, but on the source <-> amp side, not the headphone output.
Here's what somebody wrote as a reply to a review of their attenuators. I thought it's quite funny:
"Rothwell ...: Ferrari's too fast? Run it with hand-brake."
Edited by xnor - 1/17/13 at 6:08am