I have a TS-520 that I bought recently and it has VERY low output. It had been sitting for a LONG time! I removed the tubes to test them and the 12BY7A tests good. I assume that is the driver. The power tubes are S2001A. I assume they are the same as 6146. Am I right? Will 6146A or 6146B be a good sub if so? TIA David....kf8ii