I am not a fan of the Hegel Integrated amps as I find all of their amps to sound very bland and cold. It sounds like you enjoy the Hegel sound so my advice may be irrelevant. Admittedly I have only tried the 160 & 360 in my system and the issue may be with the internal DAC. Also, The Mimas with the DAC is $8,200. The 590 is around $12K.
The Mimas is extremely musical and plays very well at low volume as well as cranked, in my system. I listen mostly to classical, folk and rock music. Aesthetix also offers great service and support as well.
With that said, you should listen to them in your system and see which is a better sound to you. I know quite a few people who own and love Hegel amps so it's a matter of listening priorities and taste. I understand that the DAC in the 390 & 590 are substantially better than the previous generation. You didn't mention the speakers you will be using and that will impact the outcome for you.
Good Luck!