The HD558 comes with black rubber tape on the inside of the cups, covers about 40% of the air holes, you will want to remove the tape to make the HD558s more like the HD598, but it will not an exact match.
Removing the tape might void the warranty.
So might as well by used HD558s ($92.87, Amazon).
This aftermarket cable ($17.20-$19.60) makes it a little easier to use the HD558 with a 3.5mm jack.
Otherwise you should not need to change anything with that combo (HD558/Magni/D1).
One other possible combo is the AKG K612 Pro (120-Ohm) headphones, (open box, $175)
With the Sound Blaster Z (SB1500) sound card, $81
But I have not seen any feedback on a K612/SB-Z combo, but the combo makes sense.
You might also try asking in these two threads about anything gaming, see what they think of the combos.
Apparently Titanium HD is better for gaming.. anyways will that set up could get any better
buying an STX or titanium HD?