I'm not sure if our issues are the same because there were different triggering conditions, but I had a full volume glitch a month or so ago, and I use the same sound card that you do (Essence ST + H6) [I'm also building some Hypex amps, so we have a lot in common :-) ].
In my case it happened whenever I muted playback, and it turned out to be caused by some DSP settings that I had changed: I had some DSP modules set to "process independently of internal volume," which, according to Matt is a setting that shouldn't be fiddled with except for the DSP analyzer module.
Like I said, you might have a different issue, but it's worth a look, especially if you've been changing the default DSP settings or use a lot of DSP. I'd also recommend using the Unified Asus drivers rather than the factory drivers, as the factory drivers were pretty flaky for me (although they never caused transients like this for me).
If neither of those things is the issue, you might try and do additional diagnostics. For example, you could turn your amp way down (or, if its a block amp, unplug your speakers), and then try and replicate the glitch (with the DSP analyzer open so you can see what's happening). That may give you a clue.
In close to a year of use of JRiver with my DAC connected directly to my amps, the only volume glitch I had was the one I described above, and that was purely my own fault.
