I've run a Mullard GZ33 tube in my ST120 for quite some time. Great tube! I have only 1 of these and was thinking of buying a back up. The price on these is up over $100 and they are getting hard to find.
I saw some posts on the Emission Labs 274B tube that appears to be a GZ33. The Emission Labs 274B (The "B" has the pin out of a GZ33/GZ34) looks robust and the mesh plates of this tube I think would look unreal cool in the ST120.
Looking at the specs of this EML274 I think the tube would work-
Spendy bugger!!! I don't know if I'll do it... AT this point wondering if this tube would work and looking for help on that topic.
PS - Not sure if any Radio Head people are on this forum. On my Comcast service on the Palladian channel is a live/studio session that is unreal good. Look for it.