Amplifier: Dynaco ST70, original power and output transformers, DiyTube driver board modified to use 6 volt 6CG7 driver tubes, Chong power supply cap board. G.E. gray plate triple mica 5751 as voltage amplifier, RCA "cleartop" 6CG7 drivers.
Preamplifier: Eico HF-85, fully recapped. Telefunken "smoothplate" 12AX7's in line stage, connected to amp via "Tape Out" outlets to bypass tone control, allowing purer signal.
Source: JVC XL-V161 compact disc player (nothing fancy here, but it's always sounded good to me.)
Speakers: Klipsch Forte II's, Bob Crites crossovers. 99db/meter efficiency.
Interconnects, Speaker Wire: All by Blue Jeans Cable.
The Power Tubes: Genelex Gold Lion KT-66, Sino EL-34B, Electro Harmonix 6CA7, Electro Harmonix EL-34 (cryogenically treated by Cryoset), SED "Winged C" EL-34's and NOS Russian military 6p3S-e.
Each set of tubes was biased at 45mA per tube and allowed to warm up for 15 minutes after biasing was complete. Four cds were used for testing each quad of tubes: Elliott Smith's "Figure 8", Radiohead's "OK Computer", Norah Jones' "Not Too Late" and The Rolling Stones "Sticky Fingers" (2009 remaster). Our impressions were very similar on each of the quads.
Genelex Gold Lion KT-66: Probably the best overall performer here. Well rounded from bass to mids to highs. Nice wide sound stage. Had no trouble keeping up with the harder, faster music, yet delicate recreation of female vocals and piano or acoustic guitar. Nicely manufactured, you can see/feel the quality. Costly tubes but you're gonna notice the upgrade from just about anything! I would call their sound "accurate".
Sino EL-34B: These were matched and tested by Ruby Tubes. I'd have to call these my least favorite tubes here while Dan preferred them to the 6p3S-e. I found them to have a flat mid-range and a lack of high end air. Sounded compressed and rolled off. Bass was decent. Sound stage was narrow. Ping like a cheap toaster on warm up. I would call their sound "compressed".
Electro Harmonix 6CA7: Definitely sound more like 6L6 types than an EL-34. Great bottom end slam. This is the tube for rock music. Fast, balanced, wide sound stage. If the Gold Lions are out of your price range, try these. Some mechanical noise on start up. I would call their sound "accurate to bassie".
Electro Harmonix EL-34 (Cryoset): These are a good bargain EL-34. Sweet pentode highs and mids, yeah I'd call 'em warm. Bass was lacking compared to the 6CA7. Sound stage pretty wide. I'd have no problem using these daily...If all I listened to was jazz/acoustic...but I don't. I would call their sound "warm, yet pretty accurate".
SED "Winged C" EL-34: The warmest of the bunch. Construction definitely better than all of the above tubes except maybe the Gold Lions. Better bass than the EH EL-34's. These babies do some holographic high and mid range stuff that none of the others could match. We were both really impressed with the sound stage. I was sad to see my friend take 'em home...I would say that they sound "very warm".
Russian Military 6p3S-e: These are the ones manufactured by Reflektor with the coin base. Bass is lacking on these little guys for sure..but the high end sizzle these things throw out is really fun. Mid range is sweet too. They are a bargain though, and very well made. Worth the low cost of entry even if all you do is keep them as spares. I'd call their tone "bright and accurate".
I'll note that all of the tubes tested were burned in at least 150 hours, with 300 to 500 being average. Only the EH EL-34's had seen less than 150 hours, but not by much. I hope that somebody finds this interesting and please don't blast us on our testing methods too badly. It was fun!