I'm just curious, is your amp Sansui A707 or Sansui AU-Alpha 707? Reason for asking is I'm monitoring rare Sansuis that arrived here and Alpha 707 is comparatively rare outside Japan and Europe. Also, the Apha 707's power rating in the original version is 130wpc and 320 watts power consumption. If you have the 240V version released in Europe, it may have a higher power consumption figure.
I'm not a technician, more a listener, but if your measuring equipment is accurate, and it gives you over 10% variation in your voltage rating, you should file a complaint with your power company. Those figures would damage most of your appliances, not just the stereo system.