I tried GraphicsEnabler=No. Didn't make any difference to my issue. I still have no output on any of the ports.
Btw, while tinkering around with my BIOS settings, enabling/disabling PCIe/IGFX and switching the cable between the HDMI ports on IGFX and the GTX750Ti; I saw something different this one time, while my monitor was still hooked up to the on-board HDMI port.
For only that one time, my 'About This Mac' showed 'NVIDIA GeForce GTX 750 Ti 2047 MB' and 'System Report' showed details/properties about my monitor Dell S2440L when I clicked on the discrete card in Graphics/Displays.
THIS HAPPENED ONLY ONCE. I haven't been able to replicate this even after several tries and retracing of my steps. I tried all possible permutations and combinations for what I could have done, but no success. The only thing I can remember for certain is that GraphicsEnabler=No was one of the boot flags when that happened.
At this moment and every other time before, my 'About This Mac' shows 'Intel HD Graphics 4600 1536 MB' and 'System Report' shows details/properties for the connected monitor only for on-board graphics and not for the discrete GPU.
I don't know if this information is of any significance or not. Just putting it out there, hoping it gives some clue that can help resolve the issue with ASUS Maxwell cards.