Please help a No0b! Subwoofer distortion

[NeoVisionist]

My second hand E8-SUB left line-in is connected to my receiver's left line-out, and the active sub gets its power through the same wall socket as everything else in my rig, via a Monster power HTS 2600's switched circuit outlet.

It distorts horribly at reference volume, but is OK at lower volumes. The sub's volume dial at the front is set to minimum, and of the two dials on the back, the cutoff frequency is set to the third level of six levels (first level is 40hz, sixth is 150hz) whilst the phase dial is set to 0 (zero) of a maximum of 80.

What am I doing wrong? I remember matching the dials on the back to what I set-up the receiver's levels to be outputting, thinking I was doing it right. Could the subwoofer itself simply have been shagged, all along? Please help!