I would get the Valhalla 2 over the Lyr 2, assuming you plan on powering the HD-650 or other higher impedance cans ( Beyerdynamic) exclusively. It puts more mw (800) into 300 ohms than the Lyr 2, which puts in 660 mw into 300 ohms. Thus, if you are planning for high impedance cans, the Valhalla 2 is actually providing more voltage than the Lyr 2. This is why many on this thread have waxed enthusiastically over the synergy between their Senns and tube amplifiers (i.e. Bottlehead Crack, Darkvoice, Littledot, Woo, Valhalla 2, and so on).
The Lyr 2 is the better option only if you anticipate buying an orthodynamic (Audeze, HiFiMAN, Mr Speakers) or other lower impedance can (AKG) in the future, since these less efficient headphones crave current rather than voltage. Because I own the HD-650 along with headphones from AKG and HiFiMAN, I bought the Lyr instead of Valhalla. But I still hope to buy a Crack or Valhalla 2 just for my Senns alone because I know it makes the better match. MattTCG and others have said so multiple times on this thread and elsewhere. While both amps are great, V2 surpasses the L2 for your 300 and upward ohm Sennheisers. IMO of course.
