I use a total of 3 Subs, two front and 1 rear. All three for theater with an LFE input to the crossover for the front and a separate output from the processor for the rear. The 2 fronts are double duty and used for two channel also. The fronts are Mono Theater and stereo for 2 channel. I crossover the fronts for two channel at about 30hz so the music has to be load for them to turn on. If i play jazz they really never turn on. I sometimes play classical but usually not at load volume so on sometimes but mostly off. Now when it comes to rock like Pink Floyd, Ozzy or Deep Purple they come to life in spades as I play load. It does take lots of work to get them setup correctly but is worth the time. I feel subs load the room and fill in where the main speaker cannot ever with the 11" woofers in my main speakers. I used a graphic sound analyzer with and without the subs and the graph looked better with the subs on much less dips.
Why are subwoofers so polarizing?
I will admit I have never been a proponent of subwoofers in a 2 channel system. Maybe i've not heard the proper set up or the level of sub was not equal to the speaker. The last great application was over 20 years ago when I heard a Pro-Ac Tablette with a forgotten subwoofer. I'm sure in the last 20+ years the technology has improved but why are subs still mainly limited to home theater systems? As always I appreciate your time and thoughts.