Hard to compare a NAD Masters M33 to the Gryphon Diablo 120. I consider the NAD as a convenient lifestyle swiss army knife product that really doesn't excel at anything. The amp of the NAD doesn't deliver ample current to control speakers and I find it to be severely harsh sounding at moderate volume levels. While convenient, the NAD's built-in DAC / Streamer is average at best. I did find the BluOS control system and the DIRAC room correction to be nice to have's but again fit into the convenience lifestyle category.
Before purchasing my Gryphon Diablo 300 I auditioned the Diablo 120 and found it to have the same sonic footprint as the Diablo 300 with the same incredible build quality that Gryphon is famous for. The Gryphon Diablo 120 has ample power to drive your Monitor Audio Silver 7G 500 speakers and most any future speaker upgrades you may consider.
Don't be fooled by Hegel watts compared to Gryphon watts. Gryphon wattage ratings are always conservative and typically double down based on speaker resistance as you can see in their specs. Hegel's specs only state wattage at 8 ohms. Speaker resistance varies at different frequencies and rarely operate at 8 ohms. This is the area where Gryphon excels.
My Diablo didn't have the optional DAC module as I use a dCS Rossini as my DAC/Streamer. I did have the Gryphon phono module which will allow you to get rid of your Schiit Mani 2 phono stage. You'll be amazed how good a turntable can sound with a good phono stage.
I can't comment on the Hegel 590 as I've never owned or auditioned Hegel equipment. Based on others in this forum, it seems that a Hegel 590 would also be a great option.
I saw someone recommend a Boulder 866. I love Boulder and have owned many of their components. The 866 build quality is terrific but the 866 SQ doesn't really live up to high praises I have for their separate components. And you must use Roon with the 866 as the MConnect app that Boulder recommends as an alternative is horrible.
There's really no comparison between the NAD and Gryphon. The NAD feels like a plastic toy compared to the amazing build quality Gryphon provides. The Gryphon speaker terminals are a good indication of how serious of a component the Diablo is. And the Gryphon remote is what you'd expect from a $50,000 component.
If you want your last integrated, I highly recommend the Gryphon Diablo 120.