Entwistle also experimented throughout his career with "bi-amplification," where the higher frequencies of the bass sound are divided from the lower frequencies, with each frequency range sent to separate amplifiers and speakers. This allows for more control over the tone, because each portion of the frequency range can then be modified (e.g., in terms of tone, added overdrive, etc.) individually. The Versatone Pan-O-Flex amplifier used a different approach to bi-amplification, with separate amplifier sections for bass and treble but a single 12-inch speaker. The Versatone was used by well-known bassists such as Jack Casady and Carol Kaye.