In essence, important down to around 100 hz and not important below. I experimented with this a while ago when adjusting subs.
When I let my fronts play fullrange, I got a 150-200 ms better decay plot between 40-50 hz (-30 dB) than when I crossed them off at 100 hz with the ~ same frequency response, but I couldn't hear a difference. I could hear a difference further up, so concluded that it was a better trade-off to ignore decay at low frequencies in favor of cleaner mids from the fronts.
Not scientific by any stretch, but it made me more relaxed concerning the audibility of resonances in the deep.
When I let my fronts play fullrange, I got a 150-200 ms better decay plot between 40-50 hz (-30 dB) than when I crossed them off at 100 hz with the ~ same frequency response, but I couldn't hear a difference. I could hear a difference further up, so concluded that it was a better trade-off to ignore decay at low frequencies in favor of cleaner mids from the fronts.
Not scientific by any stretch, but it made me more relaxed concerning the audibility of resonances in the deep.