How do reduced resistances on monster work?

I right now able to reduce monster lightning resistance by -133% with Equilibrium + curses. I wonder if monster can get negative resistance and if it's even worth doing it in the first place. Do monster take more damage when they are lowered beyond 0% resistance?

Also I currently have 170 ish resistance on all my elements. Since I wont be able to increase them any higher, all resistances are likely be capped around 75 save lightning which hardcap in 76. Should I just instead invest these resistance point on other things and lower them to around 120-130?

To answer the first question, yes resistance can go below 0%. -60% resistance would mean that the damage taken is 60% higher than at 0%.

About having overcapped resistance, most people overcap to prevent going below max when a map has elemental weakness curse as a modifier. I believe 169% is needed to be at 75% through a high level elemental weakness map.

Thank you so much for answering my question! My effort to get more resistance is not wasted!