There's not much I can improve on this build.
The following are not increments in build quality, but they might make a better recording machine:
Get only 1x8GB of memory.
-I know you don't like it, because it's slower and it's just 8GB, but with this you save $30 and still have a free slot of RAM, if you want to upgrade to 2x8GB, if you want you can get 2x4GB and that's fine, but if you upgrade you'll have to get 2x8GB and it will be $30 more expensive.
- I know the drill with RAM, two different sticks might not work, but if you bring it to a shop, they can test which one works and give you that.
Get a lower quality PSU
-Seasonic S12II is still a Tier 2 PSU and it's very good, you won't notice any difference, it's a bit cheaper($49 for 620W right now)
If you do these things, you save about $50 which can get you another HDD, or you could just save the $50.
You could get the 6600k and a better H170 motherboard.(Which I recommend).
If you get a H170 motherboard, get the 2x4GB RAM, because H170 has 4 RAM slots.
BEWARE: Lowest price of H170 motherboards is about $70.
And don't do Crossfire, some games don't even support two GPUs, It's better to just buy a better single GPU.
Dual-GPU scaling is not perfect, even if it works, you might not get a huge performance increase.