Ungrounding the amp. That's it.
I would prefer to leave it grounded, but don't know what USB ground isolators will work with DSD. I'm eyeing the ifi iDefender.
Might be worth a try. It's not getting great reviews, so just make sure you can return it.
IF you'd prefer to have the amp grounded, I assume it's because of the safety scare. It's overblown, but if it worries you that much, there are two alternatives.
1. A pair of audio transformers between the DAC and amp.
This is the good stuff:
https://www.amazon.com/Jensen-CI-2RR-IsoMax-Isolator-Jensen/dp/B00ASVWYCS/ref=sr_1_37?ie=UTF8&qid=1492738876&sr=8-37&keywords=stereo+ground+isolator
I've kept a few Radio Shack ground isolators (transformers) in my kit for decades, but they are no longer available. They worked well, and were surprisingly good.
This is something that will work. I haven't tested it, but it's similar to the RS one.
https://www.amazon.com/Raptor-RGLI-MID-Channel-Isolator/dp/B00VO89UUS/ref=sr_1_50?ie=UTF8&qid=1492739094&sr=8-50&keywords=stereo+ground+isolator
If you use an power isolation transformer you don't have to have the amp grounded for safety, and don't have to have anything in the audio path.
https://www.amazon.com/300-Watts-AC-Isolation-Transformer/dp/B006NG977W/ref=sr_1_10?s=electronics&ie=UTF8&qid=1492737755&sr=1-10&keywords=isolation+transformer
You leave the adapter in on the amp power cord (or it will be grounded, and in this case, that's not what you want). The isolation transformer creates a safe situation without grounding because you could touch either AC wire and not be electrocuted, or even feel anything, even if you yourself are grounded. Without the isolation transformer, you will receive a shock doing that.