I wish Head-Direct included a speaker pigtail connector with the HE-6 like they did with the demo HE-6 that was out on loan. While these are certainly more efficient than the prototype, they certainly benefit from more power than the typical headphone amp. I'm using a Rube Goldberg setup for running my HE-6 off my Yamaha speaker amp.
A while back I had cut off the dual 3-pin XLR plugs from a balanced SR-325i cable, and re-terminated the phones as single ended for a buyer; But I'd kept the 4" cable remnant with plugs for later. So I took that cable remnant with the dual XLR plugs and put banana plugs on the bare wires on the cut end. I then hooked that up to my 55-watt Yamaha speaker amp. I have a really nice 4-foot APS V3 dual 3-pin XLR to 4-pin XLR cable, so that I could connect all my 4-pin headphones into any amp with dual 3-pin XLR. So I plugged that into the 4-pin XLR of the HE-6, and connected it to the adapter on the speaker terminals using a pair of female-female XLR connectors between the two male adapters. I seems kinda excessive using this much stuff between the amp and headphones, but it works and sounds great I guess since the cable length in the cheap adapter is so short.
To extend the cable to reach my listening chair without having the cable pulled too tight, I plugged in a pair of 3 meter PS Audio XLR interconnects between the adapters and it sounded pretty much the same. I was originally going to modify a spare n_maher built Mogami wire dual 3-pin to 4-pin adapter, by replacing the 3-pin XLR plugs with banana plugs so that the HE-6 could plug directly into that one adapter. But this 1-foot cable is much too short to do anything other than sit on the floor next to my speaker amp (which sits along the wall between my speakers). So I decided not to mod this adapter unless I get a decent 4-pin XLR extension cable to reach my listening chair. What I really need to do is go back through this thread and find posts with links to vendors that sell speaker-to-4-pin-XLR pigtails, and see about getting one about 8 feet long.
All in all, I get much better dynamic range and effortless sound with the cheap 55-watt amp. While my balanced SR-71b sounds better with the HE-6 than my other headphone amps, that combo is slightly brighter with them than the Yamaha amp, while the Yamaha amp sounds like a more powerful version of the EF5 but with slightly less bass. The Yamaha results in a little less micro-detail and smaller soundstage than the SR-71b, but it's not bad for a $200 home theater amp. I need to get a proper speaker adapter and spend more time listening before I can decide which combo I prefer, but in the end I may end up getting a Mini Tori amp like Skylab, in order to squeeze a little more performance out of the HE-6.
