Today I received my long awaited pair of rare vintage 1950's Sylvania 6CG7 black plate tubes for my AVA hybrid Ultra II DAC. It has transformed my system. I am running a pair of rare vintage 1950's Raytheon black plate 6CG7's in my AVA hybrid Ultra Preamp and together they produce this very musical analog sound that has deep bass, lush liquid midrange while maintaining high frequency detail. Imaging is excellent and sound stage is good. There is just a very slight decrease in sound stage and transparency but the trade off is well worth it. Even the slight grain in my Parasound A21 is improved.
I can honestly say that I think I may be completely satisfied with the sound of my system.