That's a normal experience for me. I've never experienced hum with a 12AU7, E80CC, or 12BH7A, but I have with 6SN7, some more than others. I have used 2 different adapters with 6SN7 ( a Chinese one and a Garage one) and they both performed identically. It's just low level hum and doesn't prevent me from using the 6SN7. However, I am pretty stuck on the E80CC now and don't want to use another tube to be honest (other than the 12BH7A).