For reference: My system has an elaborate subwoofer system so that LF capability is excellent. On the high end, my Maggies are as good or better than my ears. In my youth I used supertweeters.
However, there is a good case to be made for deliberate restriction of bandwidth at both the high and low end. The most simple example is the "rumble" filter for LP playback. Only some music, some of the time, has signal content at subwoofer or supertweeter frequency. Whenever there is no signal, noise will intrude. At the very least this will soak up amplifier power, and may be audibly objectionable.
Ideally the bandwidth of the playback system should match the bandwidth of the audio signal, which means that it should vary. The most sophisticated realization of this concept was the Autocorrelator Dynamic Noise filter invented by Bob Carver, and sold by Phase Linear. It cut the LF gain when there was no LF signal, and cut gain in several high frequency bands when appropriate, using clever analysis of signal harmonics.
Such a device is not necessary if you have a clean signal to start with. One application which I had, where the signal was very noisy, was the rear channel signal of a LP based matrix multichannel setup. For example, all the rumble gets routed to the rears, and because of the way that LPs are mastered, there is little or no true LF in the rear signal. Carver's Autocorrelator performed miracles for this application.
