Some of the 7.1 home theater systems support a "Zone 2" mode. That's
where 2 of the 7 amplifier channels are switched from rear surround
sound output to stand-alone analog amplifiers. The remaining system
then behaves as a standard 5.1 configuration.

That's the key to driving a home-made passive subwoofer or infrasonic
woofer. Put a 7.1 receiver into Zone 2 mode. Connect the subwoofer
preamp output to a Y-adaptor and then to the Zone 2 inputs. Connect a
dual voice coil sub to the Zone 2 speaker terminals. Set the Zone 2
volume to maximum.

Now your 5.1 system has a powered subwoofer with an electronic
crossover. It's cheaper and simpler than dedicating a component
amplifier to a sub and it's way easier than trying to tune a passive
crossover to match existing speakers. Fancy 7.1 systems have a
per-channel EQ that's really sweet for matching the sub, fronts, center,
and rears.