A reasonable expectation is that these new boxes use the same IR protocol (XMP), with the same device numbers and function numbers as the RNG boxes.

If the RNG IR signals "sort of" work, that implies that the protocol, device and function numbers are the same.  XMP does send a two part signal, and many implementations that we see here only send one of the two parts.  Perhaps the new boxes require both parts.   

So, what is a "tweak code"?  Can you provide some learns, especially one or two that work?