I think the brightness levels are just fine. There are a few utilities you can use which will sync up the monitors you have, at least as much as possible. One is called Colorific. You can set your monitors to use True Internet Color, or any number of other presets, so that you can standardize brightness levels and color balances. We've been using it for years, across several platforms and MANY different monitors, and it seems to be the best solution for this type of problem.