Best PC motherboards for USB Audio?

The best sounding motherboard is no motherboard. PC audio is a challenge, and unless you want to consider a lot of pricey stuff to compensate, you are better off investing in a dedicated streamer/network player.
Not to retype same stuff for 100th time, but in essence you want to eliminate processes running, minimize OS(think Linux)go to small linear power supply and separate your power and signal where you can. Keeping mechanical drives in another room and connect ideally by ethernet will also be a big improvement. computeraudiophile site is expert on much of the related minutia I mention, although plenty of related comments here. Cheers,
Spencer

The soundcard’s amount of jitter is actually pretty irrelevant, as any decent DAC (even cheap DACs will reduce jitter to below CD quality and better ones to below 24Bit quality. As long as it’s not a random one you found on eBay, you’ll be fine.

You know, it’s not IMHO the PC or the motherboard, but the PC’s power supply noise. Try to keep the PC’s power supply outside of your surge protection / AC noise filtering devices. Or at least put the on separate. Also if possible, experiment via the PC’s optical out as well. Kind of tricky part: Keep PC and audio on same AC circuit if using electrical based contacts.

In theory, USB is best due to the DAC’s ability to pull based on it’s internal clock, but sometimes the galvanic (electrical) isolation of the optical cable is superior.

I use a Raspberry Pi based streamer which uses a little bitty wall wart, and I keep that on a different surge protection. I should consider getting a linear supply for it instead.

As I was thinking about my streamer, I found that iFi makes a nice range of replacement wall warts you can use for your network components (router, switches, Raspberry Pi) :
https://amzn.to/2KLCzre
Fairly reasonably priced as far as Audiophile accessories go.

@sbank streaming isn't the answer to anything as you're still pulling the signal over a wire from somewhere it just moves the problem and USB vs ethernet is a religious war of disagreement that I don't want to get into. Having a dedicated HTPC driving the USB is clearly better than driving it from your main work station with all its other uses and connections but doubles the cost.
@erik_squires yes I had already planned on getting a steel case to reduce physical noise - having a good steel enclosure around the power supply does help reduce noise from that source. Moving a PC power supply outside the cabinet is very difficult with current power supplies with their short connectors for the motherboard
I also realize that the USB head that drives the connection to the DAC should not have anything else attached to it so I'll look for one with a spare internal head that I can wire the connection to.
I'll relocate my question to computer audiophile as suggested

Sorry, i did not mean to move the power supply out of the PC! :)

I meant to locate the PC power supply plug so it is on the "dirty" side of your power conditioner. The "dirty" side of the conditioner is where it connects to the wall. The "clean" side (or inside) are the power plugs on it you attach components to. If you attach a dirty component to the clean side, you risk polluting it.

Some nice power conditioners have multiple separate filter banks, which you could also take advantage of if possible.

@wqgq_641,
If you have to stick to one computer for general use and audio use, then @erik_squires suggestions regarding power supply and isolation are on point. 

However, consider if you can instead sticking with the current pc for other use and invest in a dedicated network player running linux. Moving mechanical spinning discs to a NAS in another room is absolutely a benefit in sound quality, regardless of the USB/ethernet discussion. If your library is small external SSDs might be another option(although generally too expensive IMHO).

Have you read about projects utilizing Raspberry Pi with Volumio app? They sound startling good vs. most of the PC based setups I've heard and they are super affordable. With your building skills, could be a great fit! I have a friend who's built a few that I could hook you up with for any email questions etc., if you like that idea. DIYaudio has lots of coverage on them. Cheers,
Spencer

I’m using a Raspberry Pi Logitech Media player based system, it sounds great. It's feeding my Mytek Brooklyn. I think including the 256 GB thumb drive I still paid less than $150. A bargain in these circles.

Maybe for christmas I’ll spend $50 and upgrade the wall wart to an iFi low noise model. It’s still switching, but at least the Pi side is clean.

https://amzn.to/2U5CSkR