As an engineer, I'm HPF'ing the whole system, probably around 30Hz, maybe higher if the subs are under-specced for the room. Simply put, even the big-boy subs that we know and love on the big shows aren't going down quite that low. For instance, the last show I did, I was running d&b Y Subs. They're specced to 39Hz-140Hz with a -5dB variance. These are high end subs, not their flagships sure, but even the KSL-SUB is specced for 33Hz-75Hz with the -5dB variance, and that's in the Infra setting.
Frankly, at those frequencies, all we're going to be pushing out is mud, which will cost us impact and headroom. I don't want those frequencies in my mix, making a mess of things in the low-end. I'm already fighting the room, I don't need to be fighting the band as well.