I agree with you that it’s highly probable that someone would like a different setup over the dt990 off the right synergy. However, you’re assuming that dt990 doesn’t suite that person’s taste when it could’ve been that they just never heard it with the right synergy. The same person that was surprised by the dt990 used to hate all Sennheisers that he listened to until he found a black silk baffle HD650 and his particular tube amp. That’s a different story for a different day but I think even slight differences can be all the difference that is needed to change someone’s mind.
I think the pitfall is saying that the differences between sources aren’t “worth it” and that it’s better to go to a different headphone is that at some point for everyone, it makes sense to stop changing headphones but there’s no way of knowing when that is. If we take your advice to the logical conclusion, someone might not look at sources until they reach a TOTL headphone like Susvara or Utopia. I think we all know that using a dongle with a Susvara makes 0 sense. I also know that trying to pair a lower end source and amp (i.e. schiit stack, JDS stack, topping stack, smsl stack) won’t do Susvara justice either.
So where is that happy medium where source components do make sense when it comes to building your system? My point is that since there are differences to be heard with source gear, it should always be in the discussion when building a system.
I also want to touch on the word “differences”. Obviously differences is a broad term. Differences can be a good thing or a bad thing. It’s easy to say going from and HD600 to an Empyrean will have differences but are they good differences or bad differences? I’d argue many bad differences and in that case, you’d benefit greater from spending money on upgrading your source than buying an Empyrean.
Using a blanket statement that moving to a different headphone will give you bigger differences doesn’t really mean much if someone was genuinely asking to improve upon their system. Like you said, there ARE differences between sources. For the most part, if they’re posting in a dac/amp thread, I think it’s safe to assume they’ve already started their journey and are looking for ways to improve the headphones they already have.
Words like “meaningful differences” and “worth” are very subjective. I think each person needs to make that determination on their own of what is worth it and meaningful. I would say in the beginning when you have a budget of less than $300, putting the most money towards a headphone is a great idea. Once you reach past that, I think finding synergistic source gear should always be in the discussion.
Obviously everyone’s journey is going to look different and there’s no one correct way to go about it. I’m at the point where I’ve found that expensive amplification and source can take a well designed transducer a long way.
Going back to the amp, I do hope this amp changes a lot of people’s minds when it comes to amplification. Like many of my friends joke, if someone tries a stealth or Susvara and doesn’t think there’s any bass to it, it’s really cause they’re underpowering the headphone. I think there’s much more to an amp than just power specs though. How it handles clipping, where is the best power window for the amp, how quickly it provide spikes of current, etc etc will separate a good amp from a bad amp.