I did a big shoot out with all my stereo receivers. I preferred the Pioneer SX-850 and then the NAD C 740. The Marantz just sucked. I was disappointed. I have more money in the rebuild of this amplifier and I think it looks awesome. It's definitely not going to be used for this application.
I'd love to try a nice headphone amp with these. What do you guys think of the Little Dot tube amps for the K601?
I would say just to save your money,
I doubt you will find a headphone amplifier that really improves the sound of your AKGs, over the current receivers.