Quadro K600 w/ bootscreens and weird behaviour after modding

Hello folks,

a friend of mine has a K600 and i modified the ROM in the well-known way (replaced existing EFI with modded EFI, which has correct DevID, VenID and BoardID. Result: No screen, card seems bricked. But card works fine in a PC (BIOS-compatible, no UEFI/GOP active).

So i scanned some roms from original mac-videocards and found out, that in the DCB-entries the head-bitmask for all connectors is set to 0x1, 0x2, 0x4, etc., but in pc-bios always set to 0xF, which means not defined/assigned dynamically.

I modded the head-bitmask in the vga-bios for both connectors to 0x1 and 0x2, and now the card is able to provide bootscreens.
But it works only in MP booting OSX/macOS, it isn't possible to start Windows, because until choosing Windows-volume he got blackscreen. The same weird happens when used in a PC, card is recognized, but no screen on all ports........

We tried also the following options:
-last rom indicator set to 0x80: Works in MP, as expected without bootscreen, but no screen in PC
-removed EFI-part: same as above.
-BIOS Information Table: Only difference was token 0x75 (uefi data) in pc-bios, i set it to "no operation", i guess it is not mandatory. Some pc-bios don't contain the 0x75-token, although having EFI-part.
-compared mac-/pc-vga-roms for GPIOs, Communications Control Block, DCB Flags: But found no differences or differences, which seem not relevant.

So, question to the professionals: Any ideas what i can also check? I'm sure that my modification is right, but incomplete.