They actually did compare the two. Here's the paragraph straight from the review.
I think the answer is simpler. As shown on videos of the mod, you really have to be willing to take your headphone apart and really be careful to make sure you don't snap a cable, all this for an improvement that is questionable. I'm sure your friends didn't find any difference between modded 558 and stock 598, but there are so many things to take into consideration, from hardware to sample sizes to inherent biases. Ultimately, I'm not saying the mod is a bad thing, and for those who are interested, I've included a video at the bottom and I suggest you go to this post to learn more. I'm just saying that this is something that only such a small number of people will do that Sennheiser probably doesn't care about it and it's not something I would be comfortable doing. I'm going for the Fidelio X1 anyway.