Oops, I think I'll have to go and correct a few posts on this forum as it seems I was wrong! 
(edit: I've corrected hopefully all my posts on this topic)
I own a motherboard for which I can disable the power feed to some of the onboard USB ports. If I disabled the power and connected the Devialet to such a port the Devialet wouldn't play music.
Also I bought a dual-headed USB cable, which is a USB Y-cable that on one side has a USB connector for data only and a second connector for power. Only connecting the data connector wouldn't let the Devialet play music through it's USB input.
After both tests I concluded that the Devialets need the +5V power active. But after reading your post, Mauidan, in which you stated the Devialet does work without the power active this didn't add up so I found a standard USB cable for some extra tests.
A standard USB cable has four (or five if you count the shield around the four leads)
1. VCC (+5V)
2. D- (data -)
3. D+ (data +)
4. GND (Ground)
If the cable is shielded the shield is connected to the metal case around the USB connectors.
I removed the insulating plastic of the cheap USB cable laying the leads bare and one by one I cut the leads inside while playing music. I started with the red +5V lead and music kept playing. Then cut the black ground lead, music still kept playing. Finally I cut the shield and only then the music stopped. Reconnecting the black ground lead resulted in music playback again. The Devialets look to have tied the shield/USB plug metal casing to ground, either directly or indirectly, I can't tell through this test.
Conclusion: the Devialets don't need the +5V power lead active, they need the USB ground connection active either through the shield or ground lead.
So the Auraliti/SoTM USB interface card only cuts the +5V connection and not the USB ground.
For me, except knowing the real workings now, I think this doesn't change anything as I want to isolate both the dirty PC power and ground from the Devialet so I'll keep powering it using both my JCat USB interface card plus Kingrex uPower battery supply.
(edit: I've corrected hopefully all my posts on this topic)I own a motherboard for which I can disable the power feed to some of the onboard USB ports. If I disabled the power and connected the Devialet to such a port the Devialet wouldn't play music.
Also I bought a dual-headed USB cable, which is a USB Y-cable that on one side has a USB connector for data only and a second connector for power. Only connecting the data connector wouldn't let the Devialet play music through it's USB input.
After both tests I concluded that the Devialets need the +5V power active. But after reading your post, Mauidan, in which you stated the Devialet does work without the power active this didn't add up so I found a standard USB cable for some extra tests.
A standard USB cable has four (or five if you count the shield around the four leads)
1. VCC (+5V)
2. D- (data -)
3. D+ (data +)
4. GND (Ground)
If the cable is shielded the shield is connected to the metal case around the USB connectors.
I removed the insulating plastic of the cheap USB cable laying the leads bare and one by one I cut the leads inside while playing music. I started with the red +5V lead and music kept playing. Then cut the black ground lead, music still kept playing. Finally I cut the shield and only then the music stopped. Reconnecting the black ground lead resulted in music playback again. The Devialets look to have tied the shield/USB plug metal casing to ground, either directly or indirectly, I can't tell through this test.
Conclusion: the Devialets don't need the +5V power lead active, they need the USB ground connection active either through the shield or ground lead.
So the Auraliti/SoTM USB interface card only cuts the +5V connection and not the USB ground.
For me, except knowing the real workings now, I think this doesn't change anything as I want to isolate both the dirty PC power and ground from the Devialet so I'll keep powering it using both my JCat USB interface card plus Kingrex uPower battery supply.
PS Audio P3, Shunyata ΞTRON Alpha Digital and HC/Furutech power cables, Paul Hynes SR7EHD-MR4, DIY Roon Server & Roon Endpoint running AudioLinux Headless, Phasure Lush^2 USB cable, Audioquest Diamond RJ/E ethernet, Uptone Audio etherREGEN, Mutec MC-3+ USB, Shunyata ΞTRON Anaconda Digital XLR AES/EBU, Devialet Expert 250 Pro CI, Nordost Tyr Reference LS cables, Von Schweikert VR-5 SE Anniversary Edition, Anti-Mode Dual Core 2.0, JL Audio Fathom F112. More detail here.
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