Did anyone manage to ``repare'' a buzz, ever?

I am going to assume the crossover frequency was higher before? I am going to take a guess that you have a bad filter cap in the power supply. I base my conclusion on the following: your woofer has a better low frequency response then your higher frequency drivers so when you switched your crossover to 80 Hz the signal was attenuated. I also assume you have a full wave rectifier where the ripple is at 120Hz and would sound more like a buzz then 60 Hz hum. If you have a good old VOM (non digital volt-ohm meter) you can do some rough checks. You can disconnect one end of the filter capacitors (after they have had enough time to discharge) and on the R x 1 scale [lace the leads across the capacitor the needle should move to the right very quickly toward 0 ohms and slowly move to the left toward infinity. If one of the capacitors does not do this this it is probably bad. What you are doing is simulating putting DC power across the capacitor. Whena capacitor is fully discharged it should look like a short hence the meter move toward 0 ohms. As the capacitor charges it looks more and more like an open circuit hence the meter moving toward infinity. I hope this helps as I have made some assumptions.