This is another tube interference myth that I can't think of any solution out of my head. If this is a "human antenna" situation, theoretically when you stand up, the antenna effect should be enhanced and you'll hear more interference.
Let's start the debugging process by checking out your system. Can you describe your audio system in more detail? From the source (a desktop PC?) to the HA3A amplifier including all the interconnect in between. In addition, it is very likely that your interference problem is caused by multiple events, so if you find something that reduced the scale or frequency of interference, you should keep that in place during subsequent tests.
- You have tried multiple headphone cables, do you happen to have a 6.35mm headphone cable that has a copper-shielded layer covering the conductor?
- As per your photo, you have removed the protector cage, can you try to put the protective cage back on and check whether this will reduce the amount of interference noise? If this does help, maybe you can test with a stronger shield by wrapping aluminum foil around the protective cage to further test.
- On top of standing up, have you tried sitting on the chair but moving the chair away from the HA3A? Allowing more distance from the amplifier to your headphone
- Similarly, remain sitting on the chair but find someone to stand beside you, at the position where have indicated in your first photo.
