So I had this hum issue as well on my WA22 (2nd gen.) and after trying out all sorts of things and determining that it was not ground loop or power cables, etc., I found that it was the pair of 6080 stock tubes that was supplying the hum. After replacing them, the hum disappeared. Problem solved. Although a bad tube may not solve everyone's hum issues, so FWIW.There are a lot of posts here on the thread regarding hum. It seems that either the amp design is flawed or some hum is expected or both.
Some people actually managed to achieve pitch black background here afaik, it all came down to tube combo.
I am in that journey myself right now. Instead of being frustrated I‘m enjoying this part of the adventure. I get immensely annoyed by hum as well and a completely silent background is my goal. I am enjoying the sound of the amp too much to give up. Also, I’m stubborn.
Here’s my personal experience so far:
1. It wasn’t ground loop for me (I’ve created a ground loop on purpose and then there was ground hum on top the amp hum just to test lol).
2. Power? I’ve tested the amp with different cables in different outlets in different parts of homes in literally two different continents (no joking, it was a pain to take the amp as my carry-on luggage). Nothing changed (props to Woo for making an amp with a 110-240v switch though. WHY CAN’T MORE PEOPLE DO THAT?)
3. All about the tubes. The stock ones were garbage. Not only they had hum but some also had RF interference.
4. You can sometimes pinpoint which tube is the cause of the hum. The hum is stronger on one side? Switch power tubes sides and see if the stronger hum migrates to the other channel. Do the same with the driver tubes. I can tell even in matched pair some tubes are noisier than others. But then again, I’m still refusing to buy the overpriced tubes from the Woo website and I’m getting mine from eBay, the classifieds here and such. I’ve read here that really strong tubes with good matching may impact on the hum. I’m too much of a fan of cold, analytical solid state amps to let myself spend that much on NOS tubes.
5. Some tubes may sound noisy from the box but will get quieter as they settle down after some hours of use. Do not buy, test for a few minutes, conclude it’s noisy AF and then flip them on head-fi. Give them a week or two. My U52 rectifier got considerably quieter.
I’ve just put a pair of GE 6AS7GA in the place of my matched quad GE 6BL7GTA yesterday. Sounds like a Mr. Coffee machine, but I’ll keep the amp running with headphones plugged in for a couple hours every day and then check it out just to be sure.
I’m also tired of going this vintage NOS tube way and I’m trying brand new psvane tubes next. Just out of curiosity and because I’m enjoying this quest. Before an audio gear enthusiast I was a guitar player so I can tell ya: every tube amp has its own cross to carry. Not a defect, a feature lolcan be super frustrating if you don’t enjoy the ride. Godspeed and keep sharing your experiences!
As for the 'overpriced' upgrade tubes from Woo Audio, I think their prices are fair because you are paying for NOS from a reputable dealer and the ability to return if there are issues. As for buying tubes on eBay, there are many bad products, IMHO, so you're taking a gamble on the quality. Again, my opinion.
I'm using the upgraded Tung Sol 5998 power tubes (from a tube dealer) and upgraded rectifer USAF 596 tube (from Woo Audio): the sound is drop dead gorgeous in combo with Diana Phi headphones, XI Sagra DAC, and ROON.
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