Hi darksabersan,
I don't know if quotes around resolution may give problems... Maybe try to remove them and update grub.
Have you tried modifying the
grub.cfg for the desired resolution? (Yes, it says not to edit it, but we like low level tweaking

) Just for testing (if you forget to edit it back it will be always rebuilt with configuration files parameters next time, so don't worry, or just update grub).
On my laptop I built a
grub.cfg from scratch, in which there are:
set gfxmode="1366x768"
set gfxpayload=keep
Maybe you can try to edit yours putting your values for these parameters.
You can try pastebining your
grub.cfg, anyways it can also be an hardware issue needing some tweakings into grub files to support that resolution, but i can't say that... Try to search online how to test for supported resolutions and modes.
Have fun with BackBox!
