Some headphones are not affected by impedance mismatch. But till a point, yes the HD558/598 doesn't sound that nice directly out of the SBZ.
One may feel that the extra bloat in bass is actually a good thing for the rather bass shy HD558/598, but problem is that when the mid-bass hits hard then distortion occurs. Not ear-tearing distortions obviously, but enough to annoy IMO.
On the other hand, the 32ohm Fidelio X1 doesn't suffer any distortion but made a lot of the details way too bloaty. Mids were quite blanketed.
Again, some people may like it as it gives the illusion of a deeper bass presentation but I personally don't. I like my bass clean and refined, with energy and impact.
One last thing (though it isn't a huge issue), adding an external amp through the headphone out of the SBZ means double amping. So if you are going that route, make sure to get an amp with as low as possible on THD and as clean as a signal accross the frequency range.