Yesterday I adjusted the crossover setting on my subwoofer and changed its position relative to the front wall. Let's say the crossover adjustment applied about 50 cents worth of time and effort to a $20K system. The bass response improvement may not have been as great as adding a subwoofer in the first place, but then, the cost was far far less.
However, the "law" won't necessarily apply if you just start fiddling with dials (or spending money). You first need to identify where your system needs improvement (for example, in the sounds of kick drums.)