First of all : congratulations to your HA-3A.
I had the same problem because my DAC is connected to the PC via USB and the DAC is the pre-amp for the HA-3A. SPdif does not transfer these, that is correct. And luckily you can use it. But what do you do if you want to transfer via USB? I've had this before, with other smaller headphone tube amps. As the volume increased, the noise also increased. When I wasn't working on the PC, it only happened occasionally, but when I moved the mouse I heard it buzzing and beeping at high frequencies. You think it's the wireless mouse, take a cable mouse, same problem. There are many sources of interference, power supply, processor, graphics card, fan and so on...
For a while I also thought that interference signals were only audible when there was no music playing and only when you turned up the tube amplifier very loudly, but that's not the case. You could even hear it in quiet passages of music. Especially since you can almost never turn the HA-3A up well above 9 o'clock, or let's say it's very rarely necessary.
A USB isolator was the only solution for me because I want to use DAC via USB.
There are also different devices and price ranges. But I wanted something high quality, I once had a really cheap one, but it didn't filter out everything 100%.
I use this one:
also see my last post:
https://www.head-fi.org/threads/cay...r-coupled-tube-amplifier.963983/post-17980739