Can someone recommend a tube amp for these? Basically I want to get into tube amp sound and would like a unit that is built well and will last. I don't want to get into a cheaper tube amp and then have to upgrade down the road. I guess my price range is $300-$600. Obviously would like to spend the least amount possible while getting a nice piece of gear.
Can't say for certain, as I don't have any first-hand experience -- but since the K702 has a relatively low impedance (62 ohms), I would suspect that an OTC tube amp would be better than an OTL one. A good option might be the Woo WA6 -- insanely high quality construction, and just slightly above your stated price range (currently $660).
I have tried the K702 AE with the Schiit Valhalla, which is an OTL tube amp -- I would probably avoid this pairing. The sound is a bit flat and lifeless when compared to my solid-state Asgard 2 with the K702 AEs. The Valhalla is a great amp with higher impedance headphones, but it just doesn't match particularly well with the AKGs, IMO.