It's a Sennheiser HD 650, the build quality might just be a small step down but nothing major. Same exact sound, meaning it's on the smoother side of neutral, with a somewhat closed in sound stage. For reference, I have used this amp with a Chord Mojo DAC and two amps: Schiit Lyr 3 and Garage1217 Project Ember II, both with quality NOS tubes in them.
This headphone is excellent for chamber music, good for jazz and pop too I suppose (don't care for these genres), weak for rap, so-so for pretty much everything else. It's extremely overrated in all honesty (by that I mean also the HD 600 and HD 650), it is not the majestic all-around wonder that many would have you believe; if you want such a thing in a headphone, you have to spend a lot, the bare minimum being the ZMF Blackwood/Ori unless you also demand top tier classical music performance, in which case the bare minimum becomes the Stax SR-007A.
As I said, the HD 6XX is only particularly great for chamber music. I can't really enjoy it for anything else, granted I am used to much better, more expensive gear. But for $200 the HD 6XX is a good price. No other $200 headphone is particularly great for any genre, like this one is for chamber.
Note that there is unspoken treble harshness with this headphone. I think it can probably be negated with a REALLY nice tube amp like an Apex HiFi Audio Teton, but on 99% of amps you will encounter this harshness at times. Not all the time but it's there, and can be heard in certain female vocals sometimes. An artificial tonality that can be fatiguing.
This headphone's primary competition is Massdrop's own AKG K7xx, which I've owned, and the Beyerdynamic DT 880 which I have also owned, since these are three neutral sounding open back headphones in the same price range. The K7xx has the biggest sound stage of the three, HD 6XX the smallest. The K7xx is a bit more resolving than the DT 880, can't call it between it and the HD 6XX though. The HD 6XX is more "intimate" sounding than the other two. The K7xx is the easiest to drive which is a big bonus for budget headphones like these. It's a close call between these three... although my favorite $200 and below headphones personally is the Fostex T50RP MKIII. I primarily listen to rock and rap for reference.
This headphone is extremely amp picky, being 300 ohms and sounding dull out of plenty of good amps. The safest bet is an OTL tube amp, Massdrop often has such amps from Little Dot and Darkvoice. Better yet, you can get the Bottlehead Crack 1.1 for $200, $300 with the Speedball upgrade, and this amp murders most sub $1000 OTL tube amps since Bottlehead knows what they're doing more than most other tube headphone amp manufacturers.