What are the consequences of flashing a modified rom with Knox enabled bootloader on a Knox enabled samsung phone? For instance I want to flash OmegaROM v21 on my Samsung Galaxy S4 GT-i9505. OmegaROM v21 is a modified Samsung edition of Android 4.4.2 Kit Kat.

According to the information in the xda-developers forum, the ROM contains a new Knox enabled bootloader. As what I understand by reading the related related research thread in the xda-developers forum and the information provided by Samsung, this will have the following consequences:


The bootloader will detect the unsigned Android kernel and because of that,
The Knox warranty flag will be set to 1.
On the boot screen it will be visible that the Knox warranty is void.
It is not possible to reset the Knox warranty flag
It is unlikely, that there will be a hack for resetting the Knox warranty flag because it is stored at a very low level, maybe in an eFuse or in the TrustZone of the ARM processor.
I will not be able to flash an older ROM. (Why? Because it will need an old bootloader and the new one disallows that?)
I will not be able to use Knox enabled apps.
I will not be able to use Knox enabled infrastructure.
I will not be able to use Knox enabled DRM.
I will be able to flash new and modified / unsigned custom ROMs
I will be able to use normal apps, infrastructure and DRM.


Are my assumptions correct, and will I be able to use DRM based services like Spotify, Amazon Lovefilm, Google Play Music, etc?
From everything I've read about the "Knox enhanced boot loader" , and what is and isn't possible with it, all of your assumptions seem spot on. Another route you may want to try,  is Safestrap. It's by far the safest method for installing custom Roms, as it bypasses the bootloader and there's no chance in tripping the Knox flag, unless you flash a Rom on the stock side. If you haven't heard of Safestrap,  Just a quick Google search will point you to it.