ohms, a power level that is high enough for the largest 8x10" cabinets and the largest venues (stadiums, outdoor festivals, etc.). If a player uses a separate preamplifier and power amplifier, she or he can buy a power amplifier intended for a sound reinforcement system or PA system or pick a power amplifier designed specifically for bass instruments. These preamps and power amps come in two formats: 19 inch rack-mountable units and units with their own wood or metal case. If a player uses a rackmountable preamp and power amp, these units and any effect units, such as an audio compressors, can be mounted in 19" rack mount road cases. The "bass stack" approach gives the bassist opportunity to customize the rig, mixing various models and brands of preamplifier, graphic equalizer, power amplifier and speaker cabinets as desired. As well, a failing component can be individually replaced without taking the entire rig out of service, as would occur with a combo amplifier. Some professional-grade amp heads, such as Ampeg's SVT400-PRO, have an audio crossover, an electronic filter that splits the bass signal into a low-pitched signal (which could be routed to a cabinet suited for low-pitched sounds, such as a 1x15" or 2x15" cabinet), and the middle and high frequencies to a different cabinet suited to this register (e.g., a 2x10" or 4x10" cabinet with a horn-loaded tweeter). Amps with a crossover can either have a single crossover point pre-set at the factory (e.g., 100 Hz) or a knob is provided to enable the bassist to select the frequency where the bass signal is split into low and higher-pitched signal. Amps with an adjustable crossover point can enable bassists to fine-tune their speaker output for a particular venue. Amplifier technology Amplifiers may be based on tube ("thermionic" or in the UK, "valve") or solid state (transistor) technology, or hybrid designs that use both technologies, typically by pairing a tube preamplifier with a transistor power amplifer. Tube amplification Vacuum tubes were the dominant active electronic components in bass amplifiers manufactured from the 1950s until the early 1970s. Tube amplifiers for bass almost always use class AB1 topology for efficiency reasons. Many bass players believe that tube amplifiers produce a "warmer" or more "natural" sound than solid state amplifiers when lightly or moderately driven, and more pleasing distortion characteristics when overdriven. Some performers also believe that tube amps have a greater level