Well it's your money.
Calling the buyer's remorse now.
In case you didn't know the i7-6700K is pretty confirmed 4.0Ghz base clock.

Antec HCG 520W semi-modular
non-modular option: Antec Neo Eco 520W

400-450W would be fine, even with a decent overlclock on both CPU and GPU, but I can't find any. AUZ ...
For SLI + high OC or fully modular or just generally one of the best PSUs: EVGA 750 G2

About the other parts:
Do you really need a 4790K?
How serious are you about overclocking?
Imho Aio liquid coolers don't make a lot of sense, they're larger, noisier and more expensive than high end air coolers and not even cooler except in some very special cases.
Do you need any of the VII Hero's features?
RAM brands are pretty much irrelevant. Do you need/want 16GB? You should be fine with 8GB, especially if you upgrade in 3 years anyway, but I doubt it's going to affect your budget much going for 16GB.
Strix is so average it's bad. The average cards it trades blows with are cheaper and the ones for the same price are simply better in pretty much or worst case absolutely everything. I just can't find a reason to recommend it. I'd say MSI Gaming 4G or Gigabyte G1 Gaming.

Apart from the usual "could've got cheaper parts with the same performance" nagging that's a nice build.
One thing though: What is up with that PSU? I mean there's no way in hell that would even touch 500W fully overclocked, so what's the reason for a 860W PSU?

Xonar DG/DGX.
DGX is PCI-E, but your motherboard got PCI aswell and the DG (PCI, otherwise identical) is cheaper.

How did you figure your onboard is dying though?

Guys, the mystery is solved.
He is salty because he bought a bad USB sound card.

You can't be this dense.

Your argument proves that the steel rims on a 20k$ car are better than 400$ alloy rims because 20k > 400. Doesn't make much sense, does it?

The ALC898 is on the Z77 Extreme4 aswell. 135$. The Z77 chipset alone costs 48$. And then there's still a lot of other hardware you need for a mobo.

Believe it or not the budget for audio hardware is higher in a 5$ USB sound card than in a typical motherboard.
I'll do a teardown if you send me one.

#34
Do you really think they'd put 1$ audio chips on a 20$ mobo?
Do you really think the chips on high end mobos are worth more than 1$?
Let's test your theory:
http://www.asrock.com/mb/Intel/Z77%20Extreme11/
400$ mobo
Realtek ALC898.
Let's how much that thing costs:
http://www.alibaba.com/product-detail/-New-Original-ALC898_60206816836.html
0.1$
10 cents.
10 fucking cents.
And you're telling 50+$ USB sound cards can't beat that?
What kind of magic did Realtek put in there?

I should probably stop posting, if you still don't get it you're either beyond any help or trolling.

Also good luck finding a PCI soundcard for less than 5$.

EDIT:
#31
Of course there is absolutely no way a 5$ USB sound card can beat a 1$ onboard chip.

PCI sound cards are obviously more expensive because they are better, not because a PCB three times the size is more expensive to manufacture.

EDIT2:
OP:
Modmic on massdrop again, you might be able to get it and a decent sound card and still stay within budget.

#509
Imho overclocking isn't worth it.
The fps shouldn't be a problem.

mATX:
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Xeon E3-1231 V3 3.4GHz Quad-Core Processor (£198.62 @ More Computers)
Motherboard: ASRock B85M Pro4 Micro ATX LGA1150 Motherboard (£56.24 @ Amazon UK)
Memory: Crucial Ballistix Tactical 8GB (2 x 4GB) DDR3-1600 Memory (£45.77 @ Amazon UK)
Storage: Samsung 850 EVO-Series 250GB 2.5" Solid State Drive (£77.90 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£37.80 @ Aria PC)
Video Card: MSI GeForce GTX 970 4GB Twin Frozr V Video Card (£269.99 @ Aria PC)
Case: Cooler Master N200 MicroATX Mid Tower Case (£33.54 @ Aria PC)
Power Supply: be quiet! 400W 80+ Bronze Certified ATX Power Supply (£39.98 @ Novatech)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer (£10.06 @ CCL Computers)
Total: £769.90
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-06-05 16:12 BST+0100

ATX:
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Xeon E3-1231 V3 3.4GHz Quad-Core Processor (£198.62 @ More Computers)
Motherboard: ASRock Z97 Anniversary ATX LGA1150 Motherboard (£57.77 @ Scan.co.uk)
Memory: Crucial Ballistix Tactical 8GB (2 x 4GB) DDR3-1600 Memory (£45.77 @ Amazon UK)
Storage: Samsung 850 EVO-Series 250GB 2.5" Solid State Drive (£77.90 @ Amazon UK)
Storage: Seagate Barracuda 1TB 3.5" 7200RPM Internal Hard Drive (£37.80 @ Aria PC)
Video Card: MSI GeForce GTX 970 4GB Twin Frozr V Video Card (£269.99 @ Aria PC)
Case: Cooler Master N300 ATX Mid Tower Case (£33.29 @ CCL Computers)
Power Supply: be quiet! 400W 80+ Bronze Certified ATX Power Supply (£39.98 @ Novatech)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer (£10.06 @ CCL Computers)
Total: £771.18
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-06-05 16:12 BST+0100

#510
That's not how overclocking works. You can build coolers and mobos with unicorn tears and the power consumption would increase just like it would without unicorn tears.

Please just shut up about Corsair PSUs. I'm getting tired of repeating this. Corsair doesn't manufacture any PSUs. They don't design them either. They're not doing any QC or testing. Everything is done by an OEM, Corsair has no influence whatsoever on the performance or build quality. The only thing they control is the price. And guess what, they're overpriced. Back in the day they used to sell CWT and Seasonic built units. The CWTs were very cheap, but ok and the Seasonic ones were really good (it's Seasonic afterall) and they sold them at the same or slightly lower prices than Seasonic themselves.
That's how they got their reputation. And then they got greedy. They are now trying to sell CWT units at higher prices than Seasonic and SuperFlower PSUs that beat them at every single measurement imaginable. They managed to put a PSU on the market that needed two revisions before it even worked.
Recommending an older, slower CPU that's more expensive to save money? Great advice!
There's no way you're going to justify a 50£ difference for 16% more effective speed. It's on sale in the UK right now, but in the US where it's 25% more expensive or whenever the budget actually matters, it's just not a good idea.

#20
Realtek ALC898.
It's not ideal but it should work.
The ModMic 3.0 used to have problems with certain driver versions on the ALC898, but I'm not sure if that's still the case for the 4.0.
There will also be a faint white noise in the background.

Do you know which chip? Otherwise just tell me which mobo, I'll find out.

Please read properly. I didn't say anything about a preamp. I said onboard might not be able to handle it.
Literally any soundcard will have absolutely no problems, it's just some of the low end onboard realtek chips.

The ModMic produces an analog signal and you can bet your ass that analog signal goes through an amp before going into the ADC. It's called preamplification because it's done before the signal processing.

Keep in mind that the ModMic needs preamplification, onboard might not cut it.

It's a pain in the ass to even diagnose that without being on-site (frankly it would still be even then) and even if I can figure out what's causing it, it doesn't mean it can actually be fixed at all.

If you have the option to switch to wired, do so.

If not, here's some preliminary questions:
Is there anyone else on the same WLAN?
Are there other WLANs that might be interfering?

So you actually listened to them beforehand?
Well shame on me for assuming otherwise then.

I'm sorry but I still don't understand. In what world do closed headphones have better bass? Hell, in what world are closed headphones better than open headphones at anything except isolation?
I mean look at this graph:
http://graphs.headphone.com/graphCompare.php?graphType=-1&graphID[]=2131&graphID[]=2751&graphID[]=2241&scale=15
You definitely didn't choose the DT770s over the DT990s for more bass, because that's not the case.
http://graphs.headphone.com/graphCompare.php?graphType=3&graphID[]=2131&graphID[]=2751&graphID[]=2241&scale=15
Large driver movement (aka bass) getting choked on the DT770s by the pressure buildup inside the isolation, as it is expected for closed headphones. So it's not better bass either.

There are few legitimate reasons to chose the DT770s over open headphones if not only you don't need the isolation, but it's actually a drawback for you.

EDIT: I assumed (I know, I know) that you meant open-back with "open-air".