Direct copy pasta from the HD650 thread of my impression of the Groove:
Used the HD650 all afternoon with the stock tubes in the Gungnir/Mjolnir 2, they sound wonderful. Directly comparing switching back and forth with Apogee Groove, they sound very close. Surprisingly, the Apogee groove sounds better to me. Mjolnir 2 with stock tubes vs Apogee Groove, Groove wins.
Treble is better, sound stage is a bit better, sounds bigger. Base is a bit less, barely noticeable, very hard to tell apart the difference. Mids are almost indistinguishable. Both sound extremely amazing. The Apogee Groove sounds like they "opened up" the headphones.
This also makes sense, as Mjolnir 2 with stock tubes is...tube. I think tube is just a smoother more relaxed sound while solid state is much more dynamic and punchy, but I don't need to tell you experts that. Very interesting comparison. Very hard to hear the difference, the thing that jumped out at me the most is how the treble sparkled much more with the Apogee Groove, but not harsh at all. No wonder Sennheiser bundled it together. Great combo!!!!
Hates off to Apogee to make a device that can pretty much go toe to toe with the big boy Mjolnir 2. Im impressed, and am very happy with both despite the Mjolnir 2 not sounding significantly better than the Groove, it can do things for the HD800S that the Groove cannot, but that requires a bit more listening and direct comparison.
I am going to post this now so I have this opinion in text so I can refer to it. If I find time, I will compare Apogee Groove vs Mjolnir 2 with the LISST tubes.
so I pulled the trigger to order the Black Friday combo HD650 + Apogee Groove from Sennheiser UK, it is supposed to be with me on this Tuesday. Hope they work fine to my ears then I do not have to return. What I really want to ask is whether anyone has used the Groove with the Apple Lightning to USB 3 camera adapter to stack with an iPhone yet?
