3rd party Zobels can work work... but the biggest expected improvement is with tube amps and speakers with big impedance rises in the mid/treble crossover regions.

Of course, you could also buy speakers which inherently lack those peaks, like from Fritz. :-)

If you want to understand how they could work, take a look at some tube amp measurements from Stereophile, and look at the curves with a "simulated speaker load."  Those ups and downs can be flattened somewhat.