Yes the modi is the DAC and magni is the amp of course each is interchangeable with other products depending on what you fancy but I like the uniform look.
I'm using it with dt770 pros (80ohm) which aren't much more than your headphones and i can definitely tell the difference in quality (depends on your source of music ofcourse e.g flac files or tidal)
The amount of volume control is mental as well, I found using my headphones with onboard audio they wouldn't go as loud as I'd like and thats before using the high-gain switch.
This is the 3rd schiit stack I've had over the years due to various reasons but I always seem to come back due to the price/performance.
Keep in mind that the magni 3+ and heresy are completely different though but neither one would be the wrong choice.
I think most importantly for me though is it offers an upgrade path for my headphones, if I'd like some more expensive cans down the line this will cater to that and I don't need anything better amp/dac wise as in my opinion beyond this is goes to diminishing returns territory, at least for me anyway.