This headphone rocks for a closed back, although I wish the low bass was clearer and tighter. Midrange balance sits very well with me, and the treble is quite clear and sparkly to boot.
I tried some SRH750 pads for the heck of it and now I like the bass much better. They will, however, mellow out and somewhat darken the sound. I do not mind for now, and I'll probably switch back to the stock pads for the heck of it later.
HD598 vs K545 strikes me as apples vs oranges or whatever. Also, while 545 is slightly V-shaped and quite bassy, I'm finding 598 very mid-forward and it boasts abundant lower mids and upper bass, which also contribute to a pleasantly thick sound. Quite a different balance from 545, not to mention it is also much more mellow than stock K545.
As much as I love HD598's balance, too much lower mids puts me off. Female vocals are way too chesty, among other things. But then I put on HD650, A2000X, T90, K601, or my modified K701 and forget all about 598.
545 also has the advantage of decent isolation, at least for me anyway. I bent the headband for a more snug fit and slightly better isolation, too.