Hah, I hope I won’t be doing that anytime soon 
Sounded like 60Hz to me 
Event Project Studio 8’s. I’ve had to replace tweeters a couple times. I love them, but more importantly, I know them
Initialy I thought they had English accents but now they sound US of A so 60Hz it is…
I have used the same pair of HR824’s for the last 10 years. The addition of some acoustic treatment to my room took even more guess work out of the equation, it tidied up the bottom end nicely, but the monitors are deceptive.
They handle to bottom end too well, I found you really have to balance the bottom end in to a respectable degree on these monitors.
Andrew