Wow you guys are gamers? I am a PC gamer myself, like Leftside I have everything on Steam. I do use a wired PS4 controller most often though, I hate when the Bluetooth connection drops or I get input lag. Taking this way off topic, but this is my PC I put together earlier this year, small form factor mini-ITX, really happy with it. Old one is now an HTPC. Running an Intel Core i7-8700K and Nvidia RTX 2080. To get the CPU temps to a happy place in such a small chassis, I removed the heatspreader from the CPU and applied liquid metal directly to the CPU dye and put it all back together. Also using a solid copper heat sink. Voids the warranty, but now cool as a cucumber. Okay, back to audio. I don't think the number of outputs and the effect on output impedance and damping factor is related to the 50/100Hz noise, that to me sounds like a grounding issue. If there is a grounding issue, your tubes may be picking up noise from the power supply, so having more tubes in place is going to amplify the noise. But looks like you've done all of the troubleshooting already and are taking steps to fix it! Hope it works out, let us know how it goes.