by higher level of quality were you looking to upgrade from the hd598 to another headphone or were you looking to add more equipment like soundcards , dac, amp etc to make the hd598 sound better?
for the first, thats hard given you're still in the budget bracket the hd598 is in. the dt990 will sound quite a bit more detailed and bright with better bass but is the complete opposite of how sennheisers generally are. its not laid back and easy listening. akg has some nice cans you may like as well but again, they arent laid back.
for the second, my choice of amp would be the o2 or magni and you have one so you're good on that standpoint. the only thing left to really improve would be the dac end of things but honestly you will probably not notice on the hd598 so its likely futile.