I wonder what it was/is, but the mid 70's Tannoys (all of them up to the Berkley at least) all had this "sting" in the high treble (measured at 9kHz in the controversial 1976 'Choice speaker book). I really am glad you've got it sorted though.
I still have a soft spot for Devons though and it may just be the driver tolerances used at the time that caused the issues you had with them. Cheviots always sounded coloued (a boxy "mehhh" kind of sound) in the shop, yet a couple of separate pairs I heard in clients homes, both with Quad amps IIRC, sounded really really good - and since these particular ones were bought and probably designed for domestic use....... The pro version was the SRM12X, wasn't it?
I can't go on and I can't go back, I don't feel so matter of fact,
I tried and tried to make good sense, What's the good to try it all again?