the cabinet in all directions. Bass combo cabinets and speaker cabinets are typically cube- or rectangle shaped. Some small- to mid-sized combo amp cabinets have a wedge shape, like a keyboard amp or a stage monitor speaker cabinet. The wedge shape, also called a "rock back" feature, enables a bassist to point their speakers up towards themselves, to make it easier to hear their sound. Tweeters High frequency tweeters, typically horn-loaded, are included in some bass instrument speaker cabinets. In the early 1980s, some performers began using two-way or three-way cabinets, with 15" woofers, a vented midrange driver and a horn/driver, with an audio crossover directing the signal to the appropriate driver. Folded-horn bass guitar rigs have remained rare due to their size and weight. As well, since the 1990s, most clubs have PA systems with subwoofers that can handle the low range of the bass guitar. The more common use of tweeters in traditional bass guitar amplifiers in the 1990s helped bassists to use effects and perform more soloistic playing styles, which emphasize the higher range of the instrument. Horns and speakers in the same cabinet are sometimes wired separately, so that they can be driven by separate amplifiers. Biamplified systems and separately-wired cabinets allowed bassists to send an overdriven low-pitched sound to the speaker, and a crisp, undistorted high-pitched sound to the horn. Since the 1960s, some bassists have obtained a similar result by plugging their bass into both an electric guitar guitar amp and a bass amp. This approach does not use a crossover, but since an electric guitar amp will only produce pitches down to about 80 Hz, the guitar amp reproduces the mid- to high frequencies and the bass amp reproduces the low frequencies. With this arrangement, distortion and other effects can be applied to the guitar amp without affecting the solidity of the bass amp tone. Some bass amplifier combos have a "whizzer cone" attached to the low-frequency woofer's centre. The whizzer cone is about the same size as a dust cap, although it resembles a miniature speaker cone. This handles the upper frequencies that are too high for the woofer. Controls, jacks and indicator LEDs Controls There are two main types of controls on bass amps: switches and rotary knobs. The simplest, least expensive practice amps and combo amps may only have a few switches and knobs, such as an "on/off" switch, a