I would say the RM800 is equally as good sounding with slightly less EQ adjustments.
The ABM600 does have a bit more weight to the sound and warmth with the tube drive but it’s probably not that noticeable in most situations
My RM800 is whisper quiet, the ABM has a bit more fan noise from its dual fans but nothing crazy
Both are great choices of amp
The RM500 is a good amp too