Well, my EF5 in high gain has pretty decent power for HE-6, and normal listening last night was at 11:30 - 12 o'clock, while loud listening is at 1:30 - 2:00 o'clock. Anything above that I can't listen to for long, although it does start to distort as I get closer to 3 o'clock and later on the volume knob. This combo still sounds great even out of the RCA jacks of my Marantz CD5001. At RMAF the prototype EF6 amp sounded more spacious and analytical or dry vs the EF5 which I preferred, and both had similar power levels.
It's true that it wont go as loud as my HE-5 LE or LCD-2 with the EF5, and my SR-71b will give me at least 4-5dB more volume from the HE-6 with a single ended source and balanced output. But the SR-71b it needs a nice warm sounding DAC to pair best with the HE-6, and DACport or DACmini work better with SR-71b than HDP or Pico DAC.
I do love the warm euphonic timbre and tone of the EF5 with the HE-6, which makes up for the amp's slightly smaller soundstage, decreased micro-detail or decreased power vs the SR-71b balanced amp. The transient attack with EF5 and HE-6 is actually slightly better than with my ZDT amp (both have 2 watts output at 32 ohm). I think the EF5 is still a steal, but I'll find out how the HE-6 sound with a vintage Marantz 2240 in a day or two. This 35 year old receiver is half the cost of the EF5 shipped, and the seller says it sounds better than his old Meier Concerto. It should put out about 5-6 watts into 50 ohms. I actually bought one of these new in 1977 to replace my 1976 Radioshack 16 watt receiver, although I have no memory of what ever happened to it after that.