I'm not sure what it is, but there's something dodgy about this analysis.
The Quad manuals say that an ESL is between 30 and 15 ohms from 40Hz to
8KHz. It should therefore be about 6 ohms at 18KHz.Ringing suggests a resonance or rising frequency response, which would be caused by series inductance and the ESL's capacitance, and doesn't fit with Phil's severe high frequency loss.
I suppose I'd better get measuring, analyzing and plaiting my own cables.