My 20L never has any hum at the speakers, there is a very slight hum from the power transformer on my one.
I have tried the JJ KT77 and thought they were a great tube, not brave enough to try the GL's as I have heard
many stories of them going thermo nuclear. Apparently they do sound exceptional. Of the many current production
power tubes I have tried my current personal favourite is the SED =C='s.
I have a pair of Phiaton MS400 headphones and have found the headphone jack on the 20L to be nothing short
of brilliant. No noise, hum, hiss or anything. Just great sound.
I also have Mullard input tubes. EF95, CV4010 and M8100.. I have found them all to be great. The M8100 have
been in there for 4 or so months, they sound the same as the CV4010 to me (I think they are the same tube, relabelled).