I've had the 7520's for about 3 months and had to post a few thoughts about my constant satisfaction and surprise, really. I find myself looking down at the rig in front me, shaking my head in amazement.
I've tried a few combinations of DACs and amps, but currently using the Aune T1 with an Amperex 7308 tube, out to a C&C BH2. The Aune's USB-in is being fed from a PC with JRiver thru a Teradak USB isolator/ LPS combo, which I highly recommend for those who want to hear how good the USB chain can sound.
I need isolation at work, so I have also the HP50, ZMF v1 Master, and the L2, along with a few IEMs, including the V6S. The 7520 is the most enjoyable of the set. Just amazing. I do think those other cans do better with more power (I have a Portaphile 627, but miss it when it's not at home and get tired of schlepping it around), but the 7520 and IEMs with the BH2 are real nice.
I dig these cans. And I can wear them for long periods. A true underdog classic bargain.
BTW, some people have mentioned upgrading the cable. I've had good luck with the Mediabridge.