Kudos to RUR/Ken, who lent me one of his metal base GZ34's to audition in my SWL 9.0SE. Compared to my NOS black base RCA/Mullard, the metal base version is definitely a step up in bass detail, midrange clarity, and overall tonal balance. It's incredible how much sonic differences there are between various rectifiers tubes. Thus far I've used...
1) The stock Sovtek 5AR4- sounded fairly good until it blew, but not as detailed bass as I'd like. Never had the chance to compare it head to head with the others.
2) A Ruby 5AR4 (which Dan sent as a replacement)- Perhaps the most detailed in the mids and highs, but a bit fatiguing after a while, and still lacking in bass. Perhaps it needs more time for "burn-in".
3) A TAD "select" 5AR4, which I haven't listened to much- Thus far seems nicely balanced, and "smoother" than the Ruby, but likewise still a bit flabby in the bass.
4) A NOS RCA/Mullard black base- I'd rank this one just a notch below the metal base version.
5) The metal base Mullard- Definitely the most "musical" sounding of all, but I'm not sure I'd go above $200 in a bidding war for one!
On the output side, I've compared the stock Tungsol 5687's vs. the JAN Philips, but have not noticed any significant differences between them.
Next up, I plan to have a go with tube dampers. I recently got the cheap red silicone ring types, and will likely order Herbies to try as well.
Since the Ruby and TAD lean towards the "brighter" side, I'll try the dampers there first.
Thanks again to Ken for teasing me with the sound of the metal base vintage. At least I now have a "reference" to aim for!