For clarity, the cab is homemade (not by me) from a kit (Bill Fitzmaurice BFM Omni 10.5) and the resistor was not glued down or anything and the whole thing is not soldered (it's joined up by connector blocks) so I reckon vibration eventually took its toll - the legs were left sticking out of the connector blocks.
Amp seems fine now, by itself it's happy to go loud. The cab was working too even with the missing crossover, must have been a blip when the resistor gave up.
I've ordered a replacement resistor, and I'll be glueing the bugger down this time!