oak plywood. Touring subwoofers are typically built from 18–20 mm thick void-free Baltic birch (Betula pendula or Betula pubescens) plywood from Finland, Estonia or Russia; such plywood affords greater strength for frequently transported enclosures. Not naturally weatherproof, Baltic birch is coated with carpet, thick paint or spray-on truck bedliner to give the subwoofer enclosures greater durability. Touring subwoofer cabinets are typically designed with features that facilitate moving the enclosure (e.g., wheels, a "towel bar" handle and recessed handles), a protective grille for the speaker (in direct radiating-style cabinets), metal or plastic protection for the cabinets to protect the finish as the cabinets are being slid one on top of another, and hardware to facilitate stacking the cabinets (e.g., interlocking corners) and for "flying" the cabinets from stage rigging. In the 2000s, many small- to mid-size subwoofers designed for bands' live sound use and DJ applications are "powered subs"; that is, they have an integrated power amplifier. These models typically have a built-in crossover. Some models have a metal-reinforced hole in which a speaker pole can be mounted for elevating full-frequency range cabinets. Use in a full-range system In professional concert sound system design, subwoofers can be incorporated seamlessly with the main speakers into a stereo or mono full-range system by using an active crossover. The audio engineer typically adjusts the frequency point at which lower frequency sounds are routed to the subwoofer speaker(s), and mid-frequency and higher frequency sounds are sent to the full-range speakers. Such a system receives its signal from the main mono or stereo mixing console mix bus and amplifies all frequencies together in the desired balance. If the main sound system is stereo, the subwoofers can also be in stereo. Otherwise, a mono subwoofer channel can be derived within the crossover from a stereo mix, depending on the crossover make and model. While 2010-era subwoofer cabinet manufacturers suggest placing subwoofers on either side of a stage (as implied by the inclusion of pole cups for the full-range PA cabinets), Dave Purton argues that for club gigs, having two subwoofer cabinets on either side of a stage will lead to gaps in bass coverage in the venue; he states that putting the two subwoofer cabinets together will create a more even, omnidirectional sub-bass tone. PA systems by size and subwoofer approach Aux-fed subwoofers Instead of being incorporated into a full-range system, concert subwoofers can be supplied with