I honestly don't know..and suggest you check with Nobels-maybe you can substitute a battery which has the ratings given in the manual. They're not expensive here, about £70 so even upgrading the battery makes financial sense.

Alkali Monocell?, only used to one cell giving ~1.5v

I just know it's German engineering and have heard good things of it,the only time i experienced it was some under-par busker murdering Hotel California to a disinterested shopping centre -the amp seemed ok,my ears weren't.

Good Luck.