I don't mess with the 6AS7's, I use the stock ones that came with the amp. As for 6SN7 tubes I use Raytheon VT-231 in the front row (Ralph mentioned this would be the best spot to try NOS tubes) and Sylvania 6SN7 GTB in the back row. I removed the middle row and use the shorting plugs supplied by Ralph to lower the gain of the amp. Otherwise I just use the stock Chinese tubes in that row.