Detuning, drop tuning, or intentionally tonally dropping the fundamental notes an instrument plays, is a popular practice among contemporary and experimental musicians. Creatively, it is a wondrous expansion of the limits set by traditional thought and practice. Detuning does, however, have its drawbacks. It can in worst cases rob otherwise good sounding instruments of their tonal qualities.
The first obvious affect detuning has is to lessen tension on the strings. One of the down sides to this practice is that upper transients are lost. This results in a more pronounced fundamental and a less articulate/bright overall sound. This is problematic as the vast majority of a perceived note is harmonic content and not merely the fundamental note. Additionally, sustained notes have a tendency to drop in pitch from the time they are struck or plucked until they resolve and fade. The duration of sustained notes is also compromised.
If proper tension, sustain and pitch are desired there are two fundamental solutions; thicker string gauges or a longer scaled instrument.
Alternate string gauges is by far the simplest and least expensive way to detune your instrument and maintain its proper playability. The lower the fundamental note is dropped below traditional tuning the thicker the string gauge should be. Something to be aware of, however, is that thicker strings reproduce fewer upper transients. They will be a tremendous improvement over detuned thinner strings, but the notes won't be as articulate as they could be. Additionally thicker strings also require a longer intonation depth and may not be able to be set up properly.
Ultimately the best way to get superior tone from a detuned or drop tuned instrument is to use an extended scale length. Where guitars are concerned, baritone instruments are commercially available that will allow a natural detuning of as much as three half tones. There are a variety of scale lengths to choose from.
Bassists have options as well though they aren't as plentiful. In general bassists must consider both increasing string gauges and making use of extended scale instruments to drop their fundamental a whole step or more.
The longer the scale length of the instrument the thinner the strings can be, and the greater tonal range the strings will produce.