First some qualifications. I only have ears to judge. I have little knowledge of circuit design etc. But I have spent about 15 yrs putting a very good system together which I would be happy to show to anyone. I have also learned that synergy is the key to success in putting together the magic that we all are looking for. I am not totally new to tubes. But if my system sounds good, I generally leave it alone. Okay, so that's not completely accurate... I leave it alone longer than many others. My point is that I have not all the different GZ34 tubes. But I have some. I tried SS as recommended. (Weber Copper cap) It sounded awful to me and my wife.
My system is listed in my profile. The amp is a Bob Latino VTA ST-70 with all upgrades offered. Bob built the amp in "09. I didn't realized the importance of a good rectifier tube until I got the Genalex Gold Lion. I've had this in a little less than a week. First impression.....WOW!!. The attack of the plucked strings on SRV's "Tin Pan Alley" was fantastic. Extremely sharp. Plenty of air. I was spell bound. But I wasn't expecting what came next. the bass kicked in and was at least an octave or more deeper than it was the previous day when I listened to it with a different rectifier tube.(Sovtek) It hit me in the chest more than it has ever done. I have fought hard to get the adequate amount of bass in my room for years. The room is 5000 cu ft which is a lot to fill. So this is welcomed. If you have a bass heavy system it may not be your cup of tea unless you can tame it. AAMOF, I turned back my sub last night. I've never done that before.
At first I thought my new cartridge (Zyx 4D) may be playing a big part since it digs out more bass than my previous cartridge. Its been in the system about 6-8 weeks. However, putting on a CD showed the same tendencies as what I had heard on vinyl, though maybe not to the same extent.
I am very happy with the Genalex GL GZ34. The price is $40 which puts it at the top price for the re-issued GZ34's. I had a GL in the amp before which had a very short life span
https://forum.audiogon.com/discussions/genalex-gl-gz-34-rectifier-did-i-just-get-a-bad-one-five-mont...However I suspect it may have been due to a weak tube being cryoed and used in a demanding amp. This one is not cryoed and was hand picked by Jim McShane. The sound is good enough that I would be willing to buy a new tube each year because the sound is that good. Certainly I hope that isn't the case. However, Jim (& I) believe that a good GZ34 should handle the ST-70, though that has been pointed out as the short coming of this amp. However, I got at least 3 yrs out of the original GZ34 tube. Maybe I'll get a good life from this one. Like the previous one, I will enjoy it while it lasts. Its a winner.