There’s a demo on the Andertons YouTube channel of the RM and the Original head; my personal feeling is the Original 500 head has a sweeter sounding EQ than the RM but the RM has the drive, sub and compressor which make it more versatile as a standalone head. Depends on how you do all your tone shaping I guess.
If I didn’t already have 3 million amps I’d have an Original 500 as a spare, especially at the current Andertons price.