Driving Stats too hard, mmm... this may be the very reason why Quad uses protection circuitry in all their stats starting from the old ESL 63's. They were very limited though, great imaging and soundstage depth, probably the most effective in stat design when it came to soundstage, due to their unique design.
If driving ML stats too hard, wouldn't there be some sort of distortion rating kicking in? The thing with stats is they have zero distortion, so when it plays smoothly, we tend to crank things up! Stats are not meant for this or least not the previous series. With the newer series, they're well capable of large dynamic swings and transient peaks that none of the previous versions could handle. So they've come a long way...
Nonetheless, stats aren't for blasting. They're meant for fine music reproduction. If you really wanted to blast that much, you're better off with dynamic driver types.
How loud were you actually going?
Even then, I'm thinking at least either the amplifier or speakers should have distorted... I think most probably your electrical grid had a surge that was not noticeable whiles playing too loud. All of a sudden this caused the electronics to damage... this does happen.
I've had two instances like this; in fact one of the speakers were Quads! There was a surge and it fried the top half of the stat panel (3 panels to be exact) and shut down one of the EHT trannys. The repair was dam expensive but I had to do it, in order to make them re-sellable. The other speakers were Wilson's. The previous owner went through a lighting surge in the house... fried everything on at the time, including his fridge. It burnt out the electronics in both X-over boards. Of course Wilson were willing to repair it but at a hefty cost. So the owner just got some third party to touch it up. I bought that very pair and got it fully redone upto spec and finally sold them off to a person who always wanted Wilson's. He still has them and uses Lamm Audio gear to drive them, absolutely tops!
So yes, these things can be repaired and modded to arrive at decent spec. Perhaps not the original spec but very close, sometimes to such great efforts where the modded versions surpass the original. Been there myself and done it to great success.
The question is, how far are you willing to spend? Are these Quests of sentimental value to you? And are they worth the trouble?
Only you would be able to determine that. You can get all the best advice possible but at the end of the day, that final decision is yours!
Cheers and all the best.
RJ