L-Pad attenuators look like a potentiometer, but they are two potentiometers in one, that will simultaneously increase resistance in one circuit while decreasing resistance in another. When wired into the speaker crossover circuit, they allow the crossover to “see” a constant impedance. This is necessary because a simple attenuator would cause the crossover frequency to change with changes in signal levels going to the driver. There should be plenty of discussions of these devices on the web.
In the good old days, level controls were common but a lot of people misused them and then complained about the sound and created a lot of hassle for dealers. I think that is why they fell out of favor. But, anyone here seriously contemplating upgrades is certainly capable of using them correctly. It is a matter of patient experimentation. If you have a three-way speaker, you would have two L-Pads, one on each of the two more efficient drivers so that their level can be matched to the least efficient drivers (typically the woofer). Given two controls and the overlap of sound from the drivers, adjustment can be complicated, but one’s patience will be rewarded.