Hi Al,
I am currently using the following setup (very nooby unfortunately):
Sennheiser 599s connected to a Filo E10k. I am looking for the next step up. Was thinking of purchasing the Senn 660s and a Filo k5.
As I live in Australia, pricing for things suck unfortunately. I have a budget of about $1k - $1.5k (for both headphones/DAC/AMP)
The music I listen to (lately at least) has been J-Pop and Metal.
Any ideas would be greatly appreciated!
Is it a k5 pro or regular k5? Also just wondering do you have a few stores that you can link that you can buy from reasonably?
Apologies for not specifying. It would be a k5 Pro.
Some of the stores that I can think of off the top of my head would be the following:
https://www.minidisc.com.au/ (this is where I bought my current gear from)
https://www.storedj.com.au
Amazon
Any further questions, please let me know 
What do you like about your 599, and what would you want to change?
As the 599s are my first real headphones (was using apple earphones previously) I like pretty much everything about them. Iāve decided to hand them down to a family member and thought I might as well āupgradeā in the process 
As Iām doing a lot of Working from Home now, Iād like a headset that I can use for an extended period of time without my ears bleeding. Iāve heard good things about the HD 660s as well as the DT 1990 pros.
Sorry for being vague, itās difficult for me to express what I like and donāt like into words.
Gotcha. Well the 660s would be a logical upgrade, would be pretty sweet imo, fits your music genre well