Thanks for your suggestions, which have helped me to narrow down the problem!
Although it is definitely not my idea of fun, I dismounted the Colibri from the Reed arm and put it in a SME type headshell in order to be able to try it on both my other tonearms (Audiocraft AC-4400 and FR-64S), using the same turntable and same phono amp.
In both cases no hum at all, which seems to indicate - as you suggested - that the issue is with the grounding of the Reed tonearm wiring or its cinch connectors. Or possibly the Boulder RCA-XLR adapters (which consist of female RCA's, a short length of wire and male XLR's).
I will have the tonearm wiring checked and perhaps consider to have the cable re-terminated with XLR connectors, bypassing these adapters altogether.