WYes, I plug the HDMI cable from the music server into a Meridian HD621 interface. I pull out a working plug from the HD621, insert the HDMI cable, and get video from JRiver but no audio, I suspect because there is no HDMI driver to select in JRiver.
Exasound supplies software to integrate its volume control with JRiver's. So, adjusting volume in JRiver via the PC or via JRemote or adjusting it via Exasound's remote all do exactly the same thing, which actually occurs digitally in the Exasound.
Yes, of course, it is a digital volume control, unquestionably the best kind known to man. With all due respect, the belief that an analog volume control is somehow superior is totally without objective proof of any kind, a classic myth. And, there are many well established theoretical reasons why an analog control cannot be superior to a digital one. Yes, that is off topic here, I know. Yes, I also like knobs, too. But, sound is most crucially important to me. I can get used to sliders, up/down buttons, etc. rather than rotary knobs. Digital volume controls could be implemented via rotary knobs, but that is rarely done.
The Exasound is indeed an extremely competent and very fine sounding DAC. Kal Rubinson recently reviewed the latest E38 in Stereophile and he finds it uniquely qualified among available Mch choices. I am holding on to my E28, previously also highly regarded by Kal, since it has balanced XLR output connectors, and there is not yet a balanced version of the E38. The much more expensive alternatives, like a stack of Mytec Brooklyns or a Merging NADAC are just not in the cards for me. That, and the E28 really does not leave me wanting for better sound.
Hypothetically, if building a Mch audio system, one could do it old school via an audio hardware path: player, cable box, AVR, speakers or, alternatively, player, cable box, prepro, amps, speakers, either one including PC input. I think there are economic and performance advantages to my simpler Cablecard tuner, PC, DAC, amps, speakers arrangement, with more of the functionality being in software rather than hardware.