I use the Bifrost Uber with USB Gen 2 USB but haven't noticed any noise from using USB. However, I haven't had the opportunity to try the Wyrd to see if it makes any difference, but so far I haven't noticed any problems.

Before trying any of these, if your computer has an open PCI-e slot, start with the JCAT card. Part of the problem with USB for audio is that all motherboard USB ports kind of end up in the same place, and none of these external devices can do anything about that. They can clean up the Vbus line, and the Short Block does a very good job at cleaning up the ground connection, but they can't stop interference from say the wireless controller for your USB keyboard or mouse.

As to why these things aren't built into DACs, it depends on which one you're talking about because they do different things. The Uptone, the Wyrd, and the iFi iUSB (which you missed) are all creating new 5V power for the Vbus line, taking the place of the power supplied by the initial USB port. Obviously it would be impossible for a USB powered DAC to create its own USB power, and there's no need for a self powered DAC to power the Vbus line. Self powered DACs can just power their USB inputs, and depending on the USB solution they choose to use, they can remove the need for the Vbus altogether. You can then simply cut the Vbus line, basically eliminating the need for the iUSB entirely.

The Wyrd and Uptone IIRC also affect the data stream, so they may still provide an improvement even on DACs that don't use the Vbus line, and even with cables were the Vbus line has been cut. The passive filters try to clean the existing Vbus line rather than create a new source of 5V power and some may or may not affect the data stream. The Short Block is a common mode choke, and for it to work, the DAC must NOT require any use of the Vbus line.

Because of the nature of how it works, the JCAT card will likely outperform all of these products, and improve the performance of all USB DACs.

I've never tried using something like this USB extender over ethernet:

However, if designed correctly, a set of these (type of) devices should give you galvanic isolation (something none of the other devices do) and the opportunity to inject a clean supply.

Great set of answers.  Many thanks. My MB also has optical (toslink) out, and I've tried that too.  While I cannot hear a difference between my USB and my TOSLink, in theory, is one better than the other?