Headphones

Here's some context: I got the HD 6XX for me. I had the 599 and an akg n700nc m2 before this. So, this is a large upgrade in terms of headphones. But I've been having some issues with the sound. The bass is very pillowy and weird, and it's just not clear. The treble and mids sound fine. I've narrowed it down to 2 possible reasons. Either I got a bad unit (I have yet to hear another unit, so it's highly likely that it's not), or my source is the issue. I have the FiiO K3 (old) with the 2.5mm balanced cable wired to the headphones, I'm on high gain mode with the bass toggle turned off.

Thanks for the read, apologies if I sound biased or skeptical, any help would be appreciated. You're welcome to take the piss out of me in the comments if I sound like a know it all.needs an upgrade?
If i upgrade what do i get? Should i just get an amp to pair with my dac as it has line-out or should i get a different dac/amp unit.
I'm planning on adding a sundara 2022 to my collection as well which is why I'm considering the source upgrade even more. (I've heard planars need more current which i don't think the k3 can provide, which might be the reason i don't like the sound of the 6xx).

Thanks for the read, apologies if i sound biased or skeptical, any help would be appreciated. You're welcome to take the piss out of me in the comments if I sound like a know it all.