So this is a typical grounding issue that cause psn leaking to the hp amp.
l7's sabaj a10h measurement has the same issue while amir's is fine:
台式设备 前几天收到一台耳放 Sabaj A10h ,这个品牌让我感觉哪儿似乎有听说过?但是确实没什么印象了。小小的研究了下感觉有点惊喜,因为在很小的尺寸内放进了高性能的耳放、前级、继电器音量。一百美金出头的价格(我在淘宝没找到卖的……)真的是很便宜。
www.l7audiolab.com
This is a review and detailed measurements of the Sabaj A10h headphone amplifier. It was sent to me by the company and cots US $120 on Amazon including Prime shipping. Note: without asking, the company made a generous donation to the forum prior to this review. As you can imagine, such can't...
www.audiosciencereview.com
Reviewers have different setups, so the results are different.
It's important to always have that in mind when reading reviews from different reviewers.
The PSN only happens when you measure it with a circuit that has a different ground.
I can easily reproduce such issue with any headphone amplifier by intentionally introducing a ground loop.
The noise will not be there when you listen through headphone, as you do not ground your headphone while listening.