File: /storefront/controller/pages/account/newsletter.php
Line:
$this->data['form']['newsletter'] = $form->getFieldHtml( array(
'type' => 'radio',
'name' => 'newsletter',
'value' => $this->customer->getNewsletter(),
'options' => array(
'1' => $this->language->get('text_yes'),
'0' => $this->language->get('text_no'),
) ));
You need to add logic between saved value and default and set 'value' =>
to it.