The Aural Enhancer is a passive R/C network that alters the frequency response throughout the bass spectrum. This pre-shaping is "blended" into the original signal through the Aural Enhancer Control. Exact frequencies affected are dependent on the characteristics of the instrument used. Basically a tone shaping control, the Aural Enhancer was developed to bring out the fundamental low notes of the bass guitar, to reduce certain frequencies that help mask the fundamentals and to enhance the high-end transients.
The Aural Enhancer has been featured on nearly every SWR amplifier since the company's inception in 1984; it is a trademark part of the "SWR Sound" people have come to know and love. The ultimate result is a more transparent sound (especially noticeable when slapping and popping), and it can make a "passive" bass sound "active" when set at or above the two o'clock position.
To understand how the Aural Enhancer works, think of it as a variable tone curve that changes depending on where you set the Aural Enhancer control knob. As you raise the control clockwise from the "MIN" position, you are elevating a whole range of sound (lows, mids and highs) at a variety of frequency points selected specifically because they're different than those selected for the individual tone controls.
This remains true up to about the two o'clock position. This position, a favorite for many users, brings out the low-end fundamentals and crisp highs. At the same time, it adds a little lower midrange to help cut through the band. However, if you go further clockwise past the two o'clock position, selected mids will start to drop off—specifically, a group of frequencies centered around 200 Hz. At this point and after, the effect becomes much more pronounced. However, the curves involved here are gentle, as opposed to the very extreme curves you can create by boosting or cutting the Active Tone controls (EQ).
Most significantly for basses, the Aural Enhancer will help bring out the fundamentals of your lower registers without masking them with overtones, as is possible when using the "Bass" control only. At the same time, it opens up the sibilance characteristics of all instruments without being harsh.
Obviously, numbers and curves and circuits all mean nothing compared to what you hear with your own ears. Play a chord, a repeated lick or a harmonic, and turn the Aural Enhancer control to various points on the knob to hear the effect for yourself. As always, your ears are the best judge when it comes to settings that affect the tone of your instrument.