Ah, maybe the X-Can v2 is NOT such a great match for the HE-400. I've been a little underwhelmed by the sub bass in the HE-400 after reading so much about how great it is. So, I plugged the K702-65 into the X-Can after listening to the HE-400 all morning (must have about 6 or so hours on the HE-400 already) and the Annies SMOKED 'em on the low bass. I was using the Daft Punk track "Doin' it Right" off of random Access Memories and the AKG won easily as far as extension and weight goes.
So, I was like "how can this be?". I looked up the impedance specs for the X-Can and the HE-400 and it looks like a bad match maybe?
HE-400 - 35 ohm impedance, 92.5 db efficency
K702 65th Anniv - 62 ohm impedance, 105 db efficency
X-Can V2 reported output impedance - 33 ohm
Here is a sample of a review I found that is a good description for the sound of the X-Can v2
The Musical Fidelity X-Can V2 boasts a completely new circuit design that results in further developing the highly rated Nu-Vista preamp. Musical Fidelity claims the result is a fuller, more detailed sound and the V2 is able to drive the most difficult headphone load with ease. The new X-Can V2 delivers. What strikes is a greater clarity and resolution on offer compared with the original design, along with the way the presentation loses much of the unwanted 'in-head' feel. There's a ton of timbre and texture to be enjoyed, while the tonal balance has a correctness reviewers find hard to fault. The X-Can V2 sounds glorious, large in soundstage and delicate when need be. It has amazingly fast and strong bass which delivers exciting, upbeat and balanced sound to the listening ear. The Musical Fidelity X-Can V2 also gives a dynamic, crisp, and with proper tubes, a warm presentation with great imaging, space and dimension. The sheer separation between voices and instruments is so pronounced that listeners never left feeling short changed. Match the X-Can V2 with headphones like the Sennheiser HD-600 and for most listeners, they end their search for audio bliss.