amplify the instrument enough for individual practice in a small room. Practice amps do not typically produce enough volume or low-frequency sound reproduction to be used in a band rehearsal or show. As such, they are mostly used by beginners or, when used by professionals, for warm-up or individual practice. They are more likely than full-size combo amp cabinets to have an open-back design, like an electric guitar combo amp. Some buskers playing on the street for tips may use battery-powered practice amps, a feature available on some models. Practice amps may have an auxiliary line-in jack, allowing a CD player or electronic metronome to be mixed into the output for practice purposes. As well, there is often a headphone output jack. Higher-priced practice amps may have a DI out jack, so that the preamplifier signal can be connected directly to a mixing board for a live show's sound reinforcement system or for a sound recording session. DI out-equipped units effectively turn the practice amp into a preamplifier unit. Combo amps For rehearsals, studio recording sessions, or small club performances, electric and upright bass players typically use a "combo" amplifier, which combines a preamplifier, tone controls, a power amplifier and a speaker (or multiple speakers) in a single cabinet. Smaller combo amps may be easier to transport and set up than using separate amplifier and speaker units, and as such, they are a popular choice for many bass players. Bass players in quieter, more acoustic genres may be able to use smaller, more modestly powered combo amps. Bassists who play in genres more associated with a high stage volume (e.g., hard rock or electric blues) may tend to use, larger, more powerful (in wattage) combo amps. Bass stacks For larger venues such as stadiums and outdoor music festivals, or for music genres that use bass instruments with an extended lower range and high stage volumes, bass players often use a more powerful amplifier (300 to 2000 watts or more) and one or more separate speaker cabinets (or "cabs") in various combinations, called a "bass stack". An example of the powerful, loud bass amplifier systems used in grunge is Alice in Chains bassist Mike Inez's setup. He uses four Ampeg SVT-2PRO amplifier heads, two of them plugged into four 1x18" subwoofer cabinets for the low register, and the other two plugged into two 8x10" cabinets. A bass stack may use a