Samsung Galaxy Reverb
The latest firmware for Samsung Galaxy Reverb (SPH-M950) is M950VPBOI1 with Android 4.1.2, released on September 17, 2015 for 1 region (CSC). Every official firmware build for SPH-M950, newest first: one row per build with all regions (CSC) that received it.
Each firmware build has its own page at https://samfirm.shop/build/{PDA}, where {PDA} is the build number from the list above. Example: https://samfirm.shop/build/M950VPBOI1

SPH-M840

SPH-L900

SPH-L710T

SPH-L720T

SPH-L600
You can find the firmware on the model: SPH-M950 page.
Yes, all firmware files for SPH-M950 are 100% official, original, and obtained directly from Samsung servers.
To flash SPH-M950, you need to download the appropriate firmware, put your device into Download Mode, and use the Odin tool to load the BL, AP, CP, and CSC files.
Downgrading is only possible if the bootloader version (binary bit) of the old firmware matches the current one. Always check the binary version before flashing SPH-M950.