It Would be helpful if you listed a budget?
Do you own any type of (TV or computer) screen?
Have any type or Internet access or cable TV or Direct TV, Dish network box?
Do you have any need or future plans on a computer?
To send more then 2-channels of digital audio thru S/PDIF (optical or coaxial), then digital audio must be encoded (compressed) using Dolby or DTS.The SB-Z can encode (using Dolby Digital Live) up to 6-channels (5.1), for sending thru S/PDIF, but the SMSL M6 can not decode Dolby or DTS.So the SMSL M6 can only take in 2-channels of PCM (un-compressed) digital audio.
Get a used (eBay) Asus Xonar DX sound card, comes with a nice CS4398 DAC chip.Plug your head amp into the DX card. Hifimediy has DACs, maybe something in the $42-$70 range. Schiit Modi DAC.(used off eBay?)
The Xonar DX might be giving the headphones a bloated (louder, less detailed) bass.You might consider getting an O2 (Objective 2) or Magni headphone amplifier.Maybe find something used off eBay.So keep using the Xonar DX as the DAC and head amp to drive headphones.
They both use the same 50-Ohm drivers
The HD558 comes with black rubber tape on the inside of the cups, covers about 40% of the air holes, so if you remove the black rubber tape, you should get something close to the HD598 sound.