I know a bit about BIOS modding but I don't know where I can modify startup values of certain PCI device registers, so any incorrectly configured register value can be corrected as early as possible.

Taking AMIBIOS8 for example. I'm not able to find out in AMIBCP where the registers of the system devices are being configured. At most I could find some SIO tables in the register section that I'm not able to figure out what exactly they are.

Context:

On ANOVO AIMB-865 and RUBY-9719VG2AR motherboards (both use AMIBIOS8), ISA slots do not function out-of-box for sound cards. To make ISA sound cards work properly on those motherboards, some registers in the ISA interface (Bus 0, Device 1Fh, Function 0) need to be modified from the values initially set by BIOS. While I have DOS programs created to make changes to the chipset registers so cards can work in DOS, trying to use such sound cards outside DOS can be a bit tricky.

This is what I'm currently doing for making ISA sound cards work outside DOS:
1. Boot into DOS.
2. Run the utility to make changes to the registers in question.
3. Run GRUB4DOS.
4. From GRUB4DOS choose the partition/kernel I want to boot.

Steps 2 and 3 can be configured as a minimal DOS startup option in DOS kernels that support customizing startup menus, such as MS-DOS, PC-DOS, FreeDOS, ROM-DOS, etc.