Pioneer Elite Reference loudspeaker system Driving an Onboard Powered Subwoofer
The most obvious way to drive a powered subwoofer is directly from the subwoofer output of a surround processor (ie, with a line-level signal). But there's another way. If you tap the output of the appropriate channel from the main system amplifier, you can use that signal to drive the subwoofer. The subwoofer must be designed for this application (many are), providing both line-level and speaker-level inputs.
With the Pioneer TX-F700 and TZ-C700, this is the only way to drive the built-in, powered subwoofer; in this case, the signal is tapped from the main input terminals. Because this tap is located inside the cabinet, the speaker is connected in the normal fashion from a user's perspective; it differs from a standard full-range speaker only in having an AC cord, a power switch, and a woofer-level control.
Powering one amplifier with the output from another might sound confusing, but all you're really doing is using the voltage of the first amplifier (one of the main system's channels) to provide a signal to the input of another amplifier (the subwoofer's dedicated amp). The first amplifier's power capability does not affect the subwoofer or its amplifier's performance—unless, of course, the first amplifier is driven into overload (clipping), which distorts its output and compromises everything down the line. For this arrangement to work properly, the signal from the main amplifier must contain all the bass we want to reach the subwoofer.
For this review, the system was set up for large left and right front speakers, small center and surrounds, and no subwoofer. In this configuration, the bass in the main left and right speaker feeds contains all the information required by the subwoofers, including LFE information from Dolby Digital and DTS source material. The left- and right-channel bass is stereo bass (if the program material is recorded that way), an arrangement that some audiophiles argue is superior to the mono-bass arrangement used in most home-theater subwoofers. But that issue is too complex for a pat "stereo bass is better" answer. Another time...—TJN