Thanks for your reply, Nicolas.
Actually, the line you mentioned doesn't exist exactly like that, but it's
$pageInfo->filter->order->value = $app->getUserStateFromRequest($this->paramBase.".filter_order", 'filter_order', 'a.ordering', 'cmd');
// change to
$pageInfo->filter->order->value = $app->getUserStateFromRequest($this->paramBase.".filter_order", 'filter_order', 'b.product_xyz', 'cmd');
// and
$pageInfo->filter->order->dir = $app->getUserStateFromRequest($this->paramBase.".filter_order_Dir", 'filter_order_Dir', 'asc', 'word');
// change to
$pageInfo->filter->order->dir = $app->getUserStateFromRequest($this->paramBase.".filter_order_Dir", 'filter_order_Dir', 'desc', 'word');
in the "public function listing".
Anyway, you gave me the right pointer, so many thanks for that!