In sentencing a defendant convicted of an offense under section
1201 involving a minor victim, an offense under section
1591, or an offense under chapter 71, 109A, 110, or 117, the court shall impose a sentence of the kind, and within the range, referred to in subsection (a)(4) unless—
In determining whether a circumstance was adequately taken into consideration, the court shall consider only the sentencing guidelines, policy statements, and official commentary of the Sentencing Commission, together with any amendments thereto by act of Congress. In the absence of an applicable sentencing guideline, the court shall impose an appropriate sentence, having due regard for the purposes set forth in subsection (a)(2). In the absence of an applicable sentencing guideline in the case of an offense other than a petty offense, the court shall also have due regard for the relationship of the sentence imposed to sentences prescribed by guidelines applicable to similar offenses and offenders, and to the applicable policy statements of the Sentencing Commission, together with any amendments to such guidelines or policy statements by act of Congress.