If you don't like them then send them back, also next time you get new headphones you don't need to "burn them in"
Headphones
A community for discussion around all topics related to headphones and personal audio.
Some people in this hobby really believe some bullshit don't they.
Just like you can't hear a difference with Hi-Res Lossless, you don't need to burn in headphones.
If anything, hi-res lossless will make more of a difference than burn-in.
Burn in's a myth, the closest thing to it would be your brain adjusting to a new sound signature.
Perhaps you just prefer the sound of your older headphones more? If you try the Fidelios for a bit more and still prefer your old ones, there's no harm in returning the Fidelios and sticking with your old pair, or trying other headphones if you're still set on getting something new. It's not like just because something is newer and/or more expensive, it'll automatically sound "better" to your ears.
I highly suspect the headphones are fine and your brain is just getting used to the different sound. I thought my HD6XX were crap (edit; actually, just very 'samey' compared to my other cans at the time) initially, now they are like nothing I have ever experienced anywhere else; literal velvet sound.
My X2's have decent bass but it's not in 'blow me away' territory, just a bit of a bump. As others have remarked my treble is also quite grainy compared to my other cans and that may or may not be for you; once I heard it I could never not hear it again.
I've never tried the Superlux so I can't compare to those.