It would seem that you have a 1.6 and not a 1.6b.
flashbios v3.0.3 works on all versions except for 1.6, hence the reason that you can get the backup to work.
I know from your earlier post that it looks like a 1.6b because of the ram etc but it can't be, you must have an odd one there.
You could reflash the same 256kb bank with 3.0.1 using the exact same method as you used with 3.0.3, then try to reboot from this bank. If it boots to 3.01 you have proved your flashing technique is correct and that it is a 1.6 xbox. If it doesn't boot or gives you a blank screen then you can relook at your flashing technique or maybe the way you burn your disks etc.
Another thing you could try is using an x2 bios which are compatable with an x3 so long as you set the switches to the correct bios size.
If this works then it would indicate a problem with the install, as the x2 bios does not use all of the connections like the x3 does.