Not sure which way is which, seems like "on" and "off" should be there if it's a HPF. I know the STC-9000s had a HPF switch there. Either way, it needs to be off in all cases if using cardioids. There are some applications for running omnis with HPF on but IMO it's better to do that in post.
Mics: Nak CM-300s, Nak CM-100s, CP-1s, CP-2s, AT-853s(Cards, Hypers, Omnis) CA-14s(Cards, Omnis)
Pres: CA STC-9200, CA-UBB
Recorders: Tascam DR-70D, DR-2D, Edirol R-09
ISO: 1 Teac ME-120, CP-3 Caps, AT-853 Subcard Caps