They should have QC as all Hifiman headphones I own had QC sticker on the bottom of the headband. Unfortunately that sticker doesn't mean much as the hands and minds that manually check every headphone opinionate different on what quality actually is and so my HE-4 had so much glue on the headband that I was extremely disappointed and then I realised why, partly, headphones like Sennheiser cost more as they have a proper QC.
That is true David, I agree. Although there are other factors consumers don't know as to why they cost so much more. Sennheiser's science personnel should outnumber Hifiman's 20:1 and so costs are higher which reflect on the higher product price.
Just let them play at slightly louder volume than you usually listen to. I never let them play just to burn in, I listen to them from day 1 and you will notice how they change in sound, same with the cable.
It's strange though how large headphone audio manufacturers like Sennheiser hasn't jumped on the planar magnetic train yet, they got the budget and the brains to do it and pull it off potentially better (both in technicality and in QC) than boutique companies like Audeze and Hifiman.