I finally compared the "stock" Sylvania VT-99 tubes with a pair of National Union VT-99 tubes. The NU tubes are far more dynamic, lively, spatial compared to the Sylvania having a rather dark and somewhat flat presentation. Three hours with the Sylvania was all I could handle after hearing the far more exciting NU tubes. If you're running an L6 with the Sylvania tubes, you are missing so much magic that comes with using the NU tubes.
Perhaps the Raytheon or Tung-Sol VT-99's might bring on further magic, but the NU VT-99's for now are outstanding. It is easy to sit here for hours and hours and listen to music I have heard for 10-20 years, mostly on LPs and hear so much new detail. Now that's saying a lot!