Note with the Benchmark DAC1 driving the HE-6, to get full output of the DAC1 you need to pull out the internal jumpers that reduce the gain on the headphone output, so you have 0dB on the output and not -10 (or -20). You can find the location of the jumpers in the manual online at their web site (I think the jumpers are different for the various models of DAC1).
I did this, and they sound pretty damn good with the volume at about 11:30 - 12:00, which is also very loud. However, I haven't compared to a higher powered amp yet, which I will do as soon as I get the chance. The DAC1 didn't work with the HE-6 when it was jumped to -10dB, and I did try a few "normal" headphone amps which also failed in the same way they would with K1000s, which is to say they were useless even though they rock other cans I own.
Currently (no pun intended), my K1000s are at a friend's over the hill, so I don't have access - but I've got an Audio Research VS-55 there which we used on the K1000s which sounded fantastic (prior to that I used a Portal Panache, a Red Wine Audio T Amp, and a Muse Model 100 MOSFET - the VS-55 was by far our favorite). I've also got a Schiit Lyr on preorder, and waiting for Hifiman to start selling the EF-6, and would love to try with a Decware amp (Either CSP-2/Taboo combo, or the Mini Torii),
So many amps, so little time....