Matt,
I've been working on this issue at home, and my experience has been that it tends to be output device and/or PC-specific. It may seem counter-intuitive that the output device would have an effect on the WDM driver's behavior, but that's been my observation (outlined below). I have certain PCs that don't exhibit any issues with the WDM driver at all, and certain output devices that don't seem to have the issue at all. But the "right" combination of PC and device will always exhibit the issue for me.
An example: I have an i7-4790K on Win 8 that has an Asus Essence ST PCI card and H6 daughter board. It shows the following behavior:
1) All combinations of buffer settings produce periodic pops/dropouts when JRiver addresses the ST using WASAPI and I'm using the WDM driver. Some buffer settings are worse than others.
2) Without using the WDM driver (outputting directly to the card) there are never any dropouts at all.
3) It gets worse under CPU load.
4) Outputting to the card using ASIO produces fewer dropouts (but not zero dropouts) when using the WDM driver.
I've methodically tried every combination of buffer settings and still get dropouts with the WDM driver with this hardware, especially with games. So as part of my differential diagnosis, I tried different devices and PCs:
1) If I plug a USB sound device into the i7-4790K (like an asus U7 or a Fiio e17), no more dropouts when using the WDM driver, even running intensive games.
2) And, if I take out the internal card (the Asus Essence ST) and plug it into another computer (e.g. an i7 2600K on win 7), I get no dropouts using the WDM driver with the exact same physical card.
So it seems to be an issue with specific machine/interface combinations. I've tried about 10-12 different output devices (including various onboard audio), and tested them (when possible) on four different machines. My personal experience from those tests is that internal PCI/PCI-e interfaces and onboard soundcards tend to be more prone to the issue than external interfaces, although external interfaces are not immune. Similarly, I've noticed that using an ASIO output is less susceptible to the issue than using a WASAPI output on the same device, but not all ASIO devices are immune, and obviously not all devices are ASIO capable.
My "best case" device is my Steinberg UR 824. It produces no WDM related dropouts with any of the four computers I've attached it to.
My "worst case" was the onboard audio on the Asus Z-97 MoBo with my 4790K system (which I obviously couldn't test on other machines). It had crackles/dropouts every five or ten seconds when using the WDM driver (but had no issues during normal direct playback).
So my reproduction advice is to try using internal/onboard soundcards on a few different machines with WASAPI as your output setting, ideally with a source like a video game that might tax the CPU a little bit (although I get the dropouts with plain old webstreaming, they're just more frequent with games). Obviously this is at best anecdotal, but I thought I'd offer the results of my own testing in case it helps reproduce it.