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.
They’re certainly good cans, I’ve seen it said that they were even the best money could buy when they were released in 2003 although in a hobby as subjective as this I find that to be a dubious claim.