From Manufacturer
The Little Labs IBP is a high-end, carefully constructed, analog phase-adjustment tool for combining two mic sources in situations where one microphone is out of phase, even if just a little. It is designed for discerning audio engineers who know exactly what they want to hear when it comes to combining multiple mics on a single source or when combining a DI'd signal with its miked counterpart. Use it when multi-miking any instrument, either for eliminating phase issues or adding color through phase shifting.
This device is rack-mountable—precisely speaking, it takes up 0.25 RU; you can fit four of them onto a rackmount tray. The device can accommodate mic, line, and Hi-Z signal, with the appropriate I/O for all three. You'll also find amplifier and re-amplifier outputs, making it a high-quality, buffered, and active direct box.
You can fine tune the phase adjustment between 0 and 180°. You'll find controls to rotate the phase by 90 or 180°, if that's what you need as well a center-frequency control that selects between a higher bandwidth phase adjustment, utilizing all-pass filters to shift the phase for either the lows or highs independently.