Agreed with Jimmybuckets, MIKE made some great budget recommendations (solid state), but there are definitely advantages with some higher end amps. Not to mention the world of tube amps, which offer a variety in the frequency response and other aspects of the sound such as impact, decay, and different tubes can even upgrade your sense of macro details with lower distortion. Of course, solid state amps sometimes are upgradeable too if they have customer-accessible OpAmp sockets, but most amps are not meant to be modified.
The Modi/Magni are certainly not the last word in audio: they’re Schiit’s cheap entry level gear. Still a great value to start, but compare a Modi to a Mojo (by Chord Audio) and you will hear a much more exciting and rich sound, and with time you’ll also appreciate how the Mojo is more refined and less fatiguing. Same thing with amps... I haven’t tried the Magni 3 yet personally, but I found the older Magni (and Jotunheim) a bit bright and brash. Still good amps, and pretty good synergy with the HD 650 and HD 6XX, but cheap whiskey compared to a Cavalli Liquid Carbon or even better a Liquid Crimson or Gold.
Visiting a specialty personal audio store in your city (I was surprised to find Pittsburgh, PA has a great one, Music to My Ear, since we’re not the biggest city nor traditionally known for being highly influential in music or tech) or making a trip to a CanJam or other audio meet (I definitely felt my first trip to Denver Colorado for RMAF was worth it, despite my retail wages) can let you demo gear in person, and it’s easy to hear the improvements in gear when you can test side by side.
Personally, at home I prefer the no-fuss reliability and setup of solid state amps. For a long time I happily stuck with “good enough to get by with” gear even knowing that there was better gear out there that I had heard, but one thing lead to another and a great deal here and there, now I’ve got a Sennheiser HDV 820 serving almost all my amping needs (with a PS Sprout doing speaker duties). Again, thousands of people agree you won’t go wrong with the budget suggestions Mike made here, but keep your mind open and realize upgrades are possible when you are ready for them.