Shure FP-24 = Sound Devices MixPre (early model). The early SD preamps had more input gain than necessary for high-output condenser microphones, especially when recording high sound pressure levels. So many of those models were designed so that if you switch on phantom powering for condenser microphones, it would automatically reduce the input gain by 10 dB.
That helped, though sometimes further attenuation was called for as well via external, in-line resistive pads. Those pads are a slight nuisance, but they're perfectly OK to use from a sound quality standpoint, since they reduce the microphones' self-noise and the ambient noise of the recording venue by the same amount as the useful signal, so the signal-to-noise performance doesn't suffer. (By the way, for best suppression of interference in your cables, those pads belong at the preamp inputs and not anywhere earlier in the signal chain.)
However, if you're using a non-standard powering/impedance conversion scheme and then feeding the signals into the preamp with the powering switched off, then the preamp is running at a more sensitive setting, so again there's a greater risk of input overload. In general if you find that you often have to set the gain controls on the preamp to very low settings, consider using a pair of balanced resistive attenuators at the preamp inputs.
Shure unfortunately took their simple, ultra-reliable, low-cost fixed attenuators and made them more complicated and expensive (~$45 each nowadays) than they need to be. Fortunately, some of the less expensive ones (Whirlwind for example, at half that price) are good (= well-shielded, reliable, and the series resistors on each side of the "H" pad match each other closely).
--best regards