Adding the subheading to the main menu?

I’ve created a site using the Purevision theme and I want to add subheader text below the main menu buttons. E.g.

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Unfortunately I’m having to decipher the Purevision code and I can’t seem to figure out how to call the ‘the_subheading()’ function. Here is the menu php code that the theme uses. If anyone could tell me how to insert the subheader text function that would be really really great!!

function purevision_nav() {
    if ( function_exists( 'wp_nav_menu' ) )
	wp_nav_menu( array( 'container_class' => 'navigation-menu',
			    'container_id' => 'navigation-menu',
			    'menu_class' => 'sf-menu',
			    'link_before'=> '<span>',
			    'link_after' => '</span>',
			    'theme_location' => 'primary',
			    'fallback_cb' => 'purevision_nav_fallback' )
			);
    else
        purevision_nav_fallback();
}

function purevision_nav_fallback() {
    global $purevision_options;
    $menu_html = '<div id="navigation-menu" class="navigation-menu">';
    $menu_html .= '<ul class="sf-menu">';
    $menu_html .= is_front_page() ? "<li class='current_page_item'>" : "<li>";
    $menu_html .= '<a href="'.get_bloginfo('url').'"><span>'.esc_html__('Home', 'purevision').'</span></a></li>';
    $menu_html .= wp_list_pages('depth=5&title_li=0&sort_column=menu_order&link_before=<span>&link_after='.the_subheading('<p>', '</p>').'</span>&exclude='.$purevision_options['excluded_paged_from_menu'].'&echo=0');
    $menu_html .= the_subheading();
    $menu_html .= '</ul>';
    $menu_html .= '</div>';
    echo $menu_html;
}

http://wordpress.org/extend/plugins/subheading/