So you are using the Yamaha as a prepro and to power only the center? That makes a lot more sense. It makes less sense that you would hear a difference tho because your Yamaha is doing all of the processing either way tho, so this configuration you have makes zero sense. The amp section in the Yamaha is beefier than the Sansui and whoever told you it has more RMS power or other such nonsense is just simply wrong, lol.
If there really is a significant difference I would attribute it to the fact the Sansui was recently refurbished and assume it's no longer accurate. Meaning whatever was done do it, wasn't done correctly. And seriously. If you're worried about preout voltage then just use the Yamaha by itself. Problem solved, and it has more power anyway!