If you're for sure certain in your observations there, then for some reason it seems your Surface Pro is not able to output enough power to your dongle. Different types of USB port have different specs in terms of how much "power" (I say this in quotes because I don't know the unit of variance - is it milliamps?) they can offer, and sometimes there are anomalies like some ports are charging ports that can offer more power than standard.......hmm, I don't know how much variance there can be from manufacturer to manufacturer but there are for sure some minimum standards associated with various types of USB ports. Maybe that's the deal, you're experiencing. I can't help you much with that, Google is your friend as well as knowing specifically the specs for the ports on your specific devices.
I finally left my post-surg couch-a-thon and got back to my desktop setup. It's head-smackin' time. The Hidizs dongle is back in fashion with the DCA Closed X's.
I plugged my Surface Pro 7 into the private network I use, and was disappointed by what I heard with the Closed X's whether using the desktop amp or the Hidizs S9 Pro dongle. I was getting ready to chuck the SP7 into the toilet, when I started thinking, "what's going on with it to muck up the sound? What process could be choking the USB power output on that particular machine?"
I started to try to run it in exclusive mode and make sure all enhancements were ruled out in the Windows sound device setup. Then it hit me: although I usually go online only with private networks where I don't need a VPN, I still had my VPN turned on for the SP7 (not my ThinkPad), and the VPN network can slow down at times. I closed out the VPN and just to be sure, ran Amazon Music HD in exclusive mode.*
This step opened up the sound, so that I didn't even need the high-pass EQ filter here in my office. Now, at the location where I was laid up, that network is sometimes also bandwidth-limited, so I think the high-pass filter may have helped there, as it did with my no-VPN-loaded Thinkpad
earlier.
It wasn't a USB problem or really a dongle-current-limitation. It was network bandwidth. Just closing the VPN also restored most of the 'slam factor' deficit in the headphones (although I still think the HD6XX's are even stronger there). All percussion is much more forward. Pumping and bass-suckout are pretty much gone when using the Hidizs dongle on a full-bandwidth network in balanced mode.
Yes, I'm feeling sheepish. Another grim reminder to think outside the dots. Or maybe, we were still inside the dots and I should have thought of it sooner, anyway.
Nevertheless, there may be a useful lesson for experimenting with headphones that require power or current. Bandwidth limitations can reduce headroom just like current or power demands, especially when both factors may be at work.
* Yes, I know everyone is figuring out that Amazon exclusive mode doesn't auto-switch the way it should to match the resolution of source material on playback. But I'm pretty sure that it does run in exclusive mode in the sense of carving off the audio stream from the Windows audio processor when you fully defeat shared-mode both in Windows and in the Amazon app.