Don't worry I believe you. But it's very possible that you may have been subjected to listening bias (like everyone else, even the best of audiophiles) because you assumed the drivers were the same. They are not. The guy who told you was misinformed. If you look into the forums and online website reviews, people will tell you otherwise that the drivers are different. And even the difference in the grills will change the sound. Don't forget that the HD 558/598 follow an angled driver design, a unique concept only given to another headphone: the HD 800.
It's interesting how the mind works. Two days ago I was able to completely tell apart between FLAC and 256 kbps file of this one particular music. It was like day and night. Today, I cannot tell the difference. As far as I'm concerned, my perceptions (or possibly bias) did not change. But iono. Maybe my ears are just not very responsive today. Who knows. The same thing may have happened to you.
I tried them with Binaural music from HDTracks (Chesky), and Rush' Moving Pictures. Really I still couldn't hear a difference. I tried rap, classical, acoustic, metal, rock, pop, and some other genre's.
I'll still make my stand on the drivers as that is what Sennheiser has told me and I doubt they would tell me something that isn't true. If they share different drivers why would Sennheiser sell me drivers for a different model? Why would they tell me they were the same if they weren't? I highly doubt they would give me false info or even sell me something if it was different.
I don't mind if you don't agree but I what you to know I had ample time with all four models and this is what I heard. I'm just stating exactly what I heard. Believe me I wouldn't make this stuff up.