I have come into possession of a pair of faulty ProAc Studio 118s.
Some cursory research and helpful feedback from those on PFM/The WAM have informed me it is likely an issue with the tweeter.
On the off-chance that someone happens to have a spare, working Scanspeak D2010/851300 tweeter lying around, I would be more than happy to take it off your hands at a price to be discussed in DMs.