The type of artist i listen to consist of Ailee,Beyonce,Rihanna,Adele,etc.Mainly female solo artist or kpop groups.I don't like a lot of bass,as I prefer to have better vocals over than a thumping head rattling bass masking the vocals of the song.Which of the two would better suit my needs?Sorry for asking this dumb question,I'm kinda new to the audiophile world.

I would take the ATH-AD900X.
(I own the HD558 & ATH-A900X)

Can you kind of give me some pros and cons of both?Also the reasoning for the ATH AD900x.Want to make use of my last $200 I'm gonna spend on audio related stuff.(I buy too much headphones and many amps to try them all >.<)

Not really too much in the way of negatives, for both headphones.
The HD598 sound will be a little more veiled or lets say sitting more towards the back rows of a concert, and everyone is very quiet at the concert, so you can easy hear the music.
The HD598 are the headphone i would pick if i knew was going to be using headphones, for several hours straight, less chance of the HD598 tiring out my ears.
I'm not really a fan for the HD598's color (like the color of the HD558 better).
 
The AD900X should have a little more life to the sound, so your not sitting in as far back as the HD598 would sound, but the AD900X might bleed the music a little.
To me in general, I think both headphones are good headphones.

Thanks for the help have a nice day.