Have been using this amp for a month or so. Have used the stock tubes (they don't sound all bad, but there is a clear benefit to tube rolling). I have used Thomson CSF 6080WA and RCA 6asjG in the power section, and sylvania 6sn7gt and new production Electro Harmonix 6sn7EH Gold.
Predictably the older tubes sound better, so I am knocking a half star off since you really need to invest more in this amp for it to sound it's best (pretty standard with most chinese made tube amps). The Electro Harmonix Gold Cryo (even with extensive burn in, 50+ Hours) hums, but it is getting better with further break in, and has reduced significantly since new. The Thomson tube is a real winner at a great price, having really great bass, but more of a flat, up front, in your face soundstage. The benefit of the older tubes seems to be that "tubey magic" wide and holographic soundstage. The Thompson is probably more accruate, with less distortion, but the NOS tubes just sound so good.
Build quality is solid, except for: tube sockets (cheap, spring type) and the volume knob (plastic, pulls off easy with no detent to align the volume control when reinstalling) and some poor wire routing internally (can see errant wires through the holes in the tube sockets, something a small zip tie could have solved at the factory). These minor issues knock off half a star. I did not experience any smoking as observed by other Massdrop owners.
The sound of this device is very good with HD650/HD6XX as others have stated (the only headphone I have tried with it). I have not tried the pre-amp out, but intend to as soon as I can get a power amp or a set of monoblocks to test it.
Bottom Line: This is a solid tube amp with great tube rolling potential, and build quality exceeds expectations at the price point. If you own the HD650, I don't think you can go wrong with this one. With the HD6XX, Darkvoice 336SE and some NOS Tubes, and a source with RCA Line out, you can have an incredible setup for under $500. It is amazing that this level of fidelity is available for such a modest price in 2018.