The 4-pin XLR output of the WA5 was originally designed to drive the K1000 (which is even less efficient than the HE-6), so it should have no problem handling the HE-6 in terms of sheer power. David Solomon also seems to enjoy the HE-6 with the WA5. However, Mike Ting of Headfonia apparently found it disappointing.
Since the WA5 comes with the mighty 4-pin XLR output, I decided to plug in the Hifiman HE-6 headphone to it, only to find myself with this huge “Whaattttt? Where did that $6,000 go?”
could be the tubes he was using which may have been stock tubes which are a poor choice for this amp