@mike257 - with regard to our specific line up, we have a male low bass vox with a C2 to C4 vocal range i.e. 65Hz to 260Hz, but he mostly sings in the C2 to C3 range i.e. up to 130Hz. He's not got a particularly strong vox and the bass vox is harder to hear in the mix anyway. Setting the HPF across the board at 150Hz has left his vocals sounding thin.
So I was thinking of making an exception for his vox, and lowering the HPF to 60Hz in line with what we have for the bass guitar, and as with the bass guitar giving him a 3dB boost in at around 120Hz (2nd harmonic of C2). But, unlike bass guitar, keeping that 3dB boost up across the full 120 to 240Hz range i.e. up to the 2nd harmonic of C3. In isolation his vox does sound a lot richer with the updated EQ settings, but we've not had chance to try it in a band mix and I guess the proof of the pudding will be then!
However, I wanted to run it passed you (and others more expert than me on here) before I cause chaos at a gig when I'm unlikely to have the bandwidth to figure out / remember / correct for what's causing us to sound awful!