I've briefly compared them on my Topping DX5. I use 490 Pro with producer pads and dust filter from 600 series earpads that I've cut to the same shape as 490 Pros stock dust cover, I didn't notice any sound difference from this, maybe it made them 5% cleaner, but it's most likely placebo. I did this only because L and R on stock dust cover annoyed my ears.
Comfort - 490 Pro with producer pads and 600 dust cover are noticeably more comfortable, but 550 are pretty good too, reduced clamp force is definitely an improvement compared to how I remember 560s, but headband padding was better on 560s because of that middle cutout. But anyway both 490 and 550 disappear on your head after a couple of minutes.
Sound - I think they sound pretty similar, I don't think I would be able to notice any differences without A/B comparisons and I would be happy with either of them.
So the differences are quite subtle for me, but here is what I've noticed so far:
1. Lows on 490 feel a bit stronger, maybe by 8-10%(nowhere near the difference that graphs would suggest), but on 550 lows are more controlled, precise and defined.
2. Stage seems better on 490, on some tracks it was pretty noticeable and on others I didn't notice at all.
3. Vocals seem moved forward a bit on 550, on 490 vocals seem to be on the same distance or level as other instruments.
4. Overall 550 seems a bit more detailed and clean
My overall impressions without trying to nitpick the small differences - are that I liked a couple of songs less on 550 and multiple songs I've enjoyed surprisingly more than I usually do on 490. So based on that alone - I think 550 is slightly better for music, but I still plan to keep 490 for gaming because of insane comfort and better stage.
If I'm not too lazy - I'll do another comparison again in 5-10 days, once I've listened to 550 more and got more used to it.