EL34 upgrade

The Winged C is a good choice and a definite upgrade from the three you've already tried. The Gold Lions are great tubes, but make sure they're compatible. KT-77's, KT-66's, and some 6CA7's are 7-pin tubes (originally 6CA7 was the American designation for the EL34, but a true separate tube was eventually developed as another beam tetrode). I really like the GL reissues, but I have one ST-70 that works fine with them and another modified one that's not compatible. Both the KT-77's and 66's are worth a shot.

The Shuguang Treasure 6CA7-Z's are even better, but they'll really set you back. I'm currently using these, and I even think they sound better than some of the NOS tubes I have or have used, but they'll pretty much set you back more than anything other than the really heavy hitter NOS tubes (Mullards, original Genalex, etc.).

I would strongly suggest NOS Siemens. Upscale Audio has them for $220 a quad, but you can find them for less. I actually have three quads of these that I'm not currently using if you'd like to try them.

Out of all the options and your stated price point, I'd suggest either the Gold Lions or the Siemens.

Sfar is correct about the "Siemens" EL34's. Of the three sets I have, they're branded Siemens, RFT, and National, but they're all exactly the same. I definitely should have called them RFT's to be more accurate. Upscale does have the tubes branded Siemens, but these and most other German EL34's of this era (including Telefunken) were all made by RFT. Regardless of the branding, I do feel that they're excellent EL34's.

Ok, for whatever reason I decided to just ask Jolida. They suggest EL34, 6CA7, or KT77. You can actually switch out some resistors to use KT88's if you want (I wouldn't want to, but some might). I didn't find out about the KT-66, but more than likely there's enough play in the biasing to work with those too.

There's already a highly active pre-tube discussion thread called "12ax7 vs 5751". I think you'll find it a highly active forum for pre-tube advice. Enjoy yourself. Generally many EL34's last for about the same amount of time. Some are significantly less reliable (current production Tesla/JJ seem to have a high failure rate). Most have found that many NOS tubes seem to last much longer (well maintained Mullards seem to last forever). In my guitar playing days, I found that Mullards, RCAs, RFT's, and pre-Sensor Svetlanas all seemed to last longer than JJ, EH, or Sovtek. These are just my findings. I haven't lived with the Treasures long enough to comment on their lifespan.

And yes tubes definitely have a burn in time. Even if you don't believe the sound changes, they definitely take a while before the bias will settle down. I find the better tubes tend to take a little longer to settle down.