OK, it seems like the FiveFish directions indicate setting the power supply voltage for 500 series modules around +/- 20v. In response to another question Ruel recommended +/- 18v.
Mmmm... no. The 500 series preamps don't have on-board voltage regulators and they just accept whatever the lunchbox rack supplies to it - which is +/-16V as per spec.
If you have the mk2 series, (i.e. rack version preamp), these preamps have on-board regulators that you can adjust to +/-18V. So to give it some extra buffer, you can set the PSU-2448 to +/-20 or +/- 22V. Because when this voltage arrives at the mk2 preamp, they'll be regulated down to +/-18V.
Are FiveFish x72 pre's going to be happy with 16v? I'm thinking of setting it low, at 16v, and seeing how things sound.
They'll work even with +/-15V or lower.
+/-16V will be just fine.