Thanks for the review. I have had the Valhalla 2 , by way of Bifrost DAC and Mac Pro ,1 via USB direct. I also have the AKG 702 headphones. Usual volume point is 12 o'clock position. I have also listened to these headphones via my Beresford Caiman DAC/Headphone amp sourced by RCA Toslink out to the Beresford. Files are Redbook CD's ripped lossless to ALAC or FLAC. You say you had the gain on low. I find that the bass response on the AKG's seems thin on the Valhalla compared to the Beresford. I have the gain set on Hi on the Valhalla. Does the Valhalla have the juice to run the AKG's? 

HD598 - " I find that the soundstage isn’t as wide or deep as that of the other headphones." This is why I keep putting my Senns back on the rack and keep going back to the AKGs time and time and time again. HD650 have a slighty tighter sound than the K701, but I just perfer the soundstage on the K701 so much more. I am using a SS only right now and have a Valhalla on the way. So lets see how the Senns and AKG pair with an OTL.