I've had a chance to listen to some material with different headphones using the Magni+ and Modi+, here are my thoughts:
My trusty ol' Irish-made HD-650 from the 2000s, owned them the longest of any of my headphones (and, yeah, they've had some replacement parts over the years, earpads and the headband pad, cable - but the drivers are still kicking butt for me!): they have always been a set of cans that blossom nicely when given enough power and can really expose weaknesses in amplification, so I am pleased to say the Magni+ drives them well at all listening volumes I tested, and to my ear helps the low frequency reproduction notably. Getting them out and going through some very familiar tracks reminded me of how much I love 'em, even with the HD-660 S right beside them they don't seem
deficient so much as
different. HD-660 S are faster and resolve some low frequency content better depending on the recording, but the frequency balance on the HD-650 is super well tuned, just so pleasing with so much material. I love the Sennheiser house sound. If they ever decide they're done with headphones I'm gonna be in a scramble to get backups of the 660 S and 650, they're just awesome to me. And the Schiit stack is great with them, no reservations recommending these for other Sennheiser fans.
Beyer 250-ohm DT-990 and DT-770: man, every time I get these out right after using my other cans it strikes me again just how different they sound! But, the Magni+ doesn't let them down in the least, they just have a very different voicing than my other preferred headphones. Their 250-ohm drivers are not the easiest to bring the best out of, but this little solid state amp kicks them into gear nicely. Wonder how it'd do with the old 600-ohm ones? Not gonna find out
The DT-770 have been a real favorite over the years for me, might be my preferred closed headphone and listening to them via the Schiit stack is a treat. Lows that can get muddy with poor amplification are a lot tighter which helps to get past an impression of "well that's a lot of bass" to being able to actually hear the distinct bass elements in a mix better. Good stuff here, really pleased.
AKG Q701 - I got these back when they were new, Made in Austria, I've always loved these for their strengths which I feel are disproportionately greater than their weaknesses. They never were the toughest built headphones; though the frame has hung in there just fine I've had to refurbish other aspects as time has gone on so they too have non-stock pads, cable, and I did the "rubber band mod" when the elastic bands gave up the ghost along the way (works great, head band tension is just fine, I'm sure it's in the eventual future of my K712 Pro too - maybe
don't use little tiny elastic bands on nice headphones, huh? I digress). I feel they're my best cans for throwing on to check certain critical aspects about a mix, high frequency resonances & subtle panning elements that the others might conceal when I'm working in my DAW. Curiously, getting the AKG K712 Pro didn't change this calculus for me - balancing the bass with the other frequencies was a great trick and makes them very pleasant to listen to, but the focus and presentation of the Q701 is ideal for getting the "close look" at some aspects, whereas the K712 give a fuller presentation of the whole mix. Q701 has pretty anemic bass, but it's still there - I'd probably mix bass-heavy if I relied only on the Q701, but I've got monitors and a sub (not to mention other, bassier cans!) for a better look at the big picture when I don't want its particular "view." Modi+/Magni+ sounds very exciting with the Q701 or the K712 Pro - no problem keeping up with the needs of whatever material I checked with them.
The Magni+ does not change the fundamental voicing of any of the cans I've used, which makes sense, but it seems like it helps them "do their thing" better. I seriously feel like the Modi+ improves the soundstage on these over my prior converter setup - Modi+ definitely has superior crosstalk and distortion figures, and I frankly wouldn't be surprised if the channel balance is better with this setup from converter through to amplification - maybe that's to do with it. I noticed aspects of some favorite recordings I had not heard so specifically before with the Modi+ and Magni+ stack.
Nice work Schiit!