Personally, I very much doubt that a difference could exists that would only be audible with one pair or headphones, or one model of speaker. It seems much more likely that it is audible on some and not others. This being the case, who's to say that you won't be able to hear that difference on your next pair of headphones? I would suggest that "99% of audiophiles" change equipment relatively frequently - so this isn't at all unlikely. I would even go as far as to say that, if I were to hear some combination of DAC and headphones that really sounded significantly better than the ones I currently use, I might consider switching over to them.... wouldn't you? (How is this any different than, for example, buying a new pair of speakers, then realizing that you need a better amplifier to reach their full potential?)
I think the important issue is to try and distinguish the type of "difference" we're talking about. Are we talking about two pieces of equipment which happen to specifically work very well together - which could be because they are both flawed and the flaws cancel out - in which case it is quite possible that neither will sound especially better when paired with any other product? Or are we talking about one piece of equipment that is really significantly better - but so far we can only distinguish that superiority with one other "mating" piece of equipment? In other words, if that DAC really does sound better - but only with one pair of headphones so far; then it is not at all unlikely that there are other headphones out there that will also be able to point up that difference - and, if there aren't, then there probably eventually will be.
Unless you are determined to never upgrade or change equipment, then it seems foolish to ignore a potential improvement when one presents itself.