On the advice an audiophile friend I tried a matched quad of the new Genalex Gold Lion KT66's in my personal Dynaco ST-70. They are still breaking in and only have about 50 hours on them but so far sound better than any tube I have ever put in that amp - even vintage Mullards. A really great sounding tube BUT expensive - $170 a quad plus shipping. The price should come down a little on these tubes in the next few months. Extremely well made (in Russia) tube. I looked them over inside and out with a magnifying glass. On the advice of the same audiophile I was told that they should be biassed a little higher than EL34's - about 50 - 55 milliamps per tube. I am using 50 milliamps right now. They look like a little coke bottle KT88 and the glass seems thicker than a normal tube.
Very large soundstage and good extension at both frequency extremes. Airy but smooth highs. Most listening was done in the normal pentode ultralinear mode. Nice mid range projection when running this amp in triode. Some tubes respond better than others when running an amp in triode. IMHO this is probably the best current production output tube you can use in your Dynaco ST-70, Mark II, Mark IV or any EL34 based amp.
Bob Latino
Photo at of this amp at > http://i9.photobucket.com/albums/a78/Bo ... thKT66.jpg