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From a SoundStage review:

The X-30 employs high-quality active circuitry. The high-pass section topology is third-order active Sallen-Key, implementing a Butterworth transfer function. It uses 1% metal-film resistors and 5% metal/polyester-film capacitors. The low-pass section also uses a third-order, 18dB/octave slope, and it has two mono RCA subwoofer outputs that can be run independently or together.

X-30 is a line-level device, made to work in conjunction with a stereo preamplifier#8217;s main outputs. It will also work with receivers or integrated amplifiers that have variable preamp line outputs -- not tape outputs. You can select one of three fixed crossover frequencies: 50Hz, 80Hz, or 120Hz. It has a variable pot-controlled low-pass sections that let you roll the subwoofer off at any frequency between 35Hz and 160Hz.

The other front panel controls are a 0-to-180-degree phase control and a volume control. I found the variable phase control to be a particularly useful feature. When the subwoofer is not located in the same plane as the main speakers, as will be the case in many installations, time-delay errors will occur and result in cancellations and suck-outs of other bass frequencies.

The good news is that upon inserting the X-30 in my system, I was elated to find that, contrary to my preconceptions, the usual signal degradation that occurs whenever an audio signal is passed through active circuitry was virtually non-detectable. That is to say that the soundstage was expansive, the dynamics were unbridled, and the frequency balance was virtually unaltered.

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