Meanwhile, my MSI Z87-43G motherboard boot screen doesn't display unless Iuse the built-in graphics card. When I use my Radeon R9 270X it just goes
blank, but still boots through to the default first grub menu item. 4K
Goes blank when exactly? Do kernel and initrd load first? Do you see any boot messages before a shell prompt or login greeter are supposed to appear? If Plymouth is installed, have you tried removing it?
monitor. Of course, the obvious issues with my graphics card means I don't
use it. The newer driver for Radeon cards does not support my card, and the
old Catalyst driver is incompatible with Jessie.
Did you try the driver that supports both gfxchips? Devs are trying to get away from chip-specific drivers:
http://www.phoronix.com/scan.php?page=news_item&px=Ubuntu-De bian-Abandon-Intel-DDX
In Jessie the modesetting driver is still a separate package, but beginning in server 1.17 it's built into the server. With the modesetting X driver installed and ATI and Intel drivers purged, modesetting ought to be used automatically for both chips, unless that R9 270X, which according to Wikipedia doesn't seem to exist, is too new to be supported without enabling any optional repo, or at all.