I recently picked up some Sylvania three hole 6SN7T's from Dave Hollander. I'm using them in a pair of Octave Audio Amps. The tubes supplied by the manufacturer had no markings. So, I'm not sure what I replaced but the difference surprised me.
The Sylvania's three hole tubes produced a much better layered soundstage. I also was heard sounds at the rear of the stage more clearly. Previously these were masked. Finally, the transparency had increased from the front to the rear of the stage. Overall, the differences were not subtle and surpassed those of the supplied tubes and some Tung-Sols that I tried. I highly recommend that you give the Sylvannas a try.
The Sylvania's three hole tubes produced a much better layered soundstage. I also was heard sounds at the rear of the stage more clearly. Previously these were masked. Finally, the transparency had increased from the front to the rear of the stage. Overall, the differences were not subtle and surpassed those of the supplied tubes and some Tung-Sols that I tried. I highly recommend that you give the Sylvannas a try.