Magento Blocks – Overwriting Adminhtml Block from Another Extension Doesn’t Work

I'm trying to find a solution since a couple hours now. Unfortunately without any luck, so I need to ask you for help.

I'm trying to add a download button to a grid which will be provided by an extension.
If I'm customize the concerning file in Block/Adminhtml/O directly the button shows up (Add a button to admin grid). But I don't want to customize the extension – I want to overwrite the concerning block. And this is where I got stuck. Nothing happens, it seems there is an issue somewhere but I can't find it.

Here is my code

My confix.xml

<?xml version="1.0"?>
<config>
   <modules>
       <Foo_Quotedownload>
           <version>0.1.0</version>
       </Foo_Quotedownload>
   </modules>
   <global>
      <blocks>
          <rewrite>
  <adminhtml_qquoteadv>Foo_Quotedownload_Block_Adminhtml_Qquoteadv</adminhtml_qquoteadv>
            </rewrite>
      </blocks>
   </global>
</config>

My app/etc/modules/Foo_Quotedownload.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Foo_Quotedownload>
            <active>true</active>
            <codePool>local</codePool>
            <depends>
                <Ophirah_Qquoteadv/>
            </depends>
        </Foo_Quotedownload>
    </modules>   
</config>

The original config.xml from the module (only interesting part)

<blocks>
<qquoteadv>
    <class>Ophirah_Qquoteadv_Block</class>
</qquoteadv>
<checkout>
    <rewrite>
        <onepage_shipping_method_available>Ophirah_Qquoteadv_Block_Checkout_Onepage_Shipping_Method_Available</onepage_shipping_method_available>
    </rewrite>
</checkout>
</blocks>

The original concerning Block

<?php
class Ophirah_Qquoteadv_Block_Adminhtml_Qquoteadv extends    
Mage_Adminhtml_Block_Widget_Grid_Container

My block which lives in Foo/Quotedownload/Block/Adminhtml/Qquoteadv.php

<?php
class Foo_Quotedownload_Block_Adminhtml_Qquoteadv extends   
Ophirah_Qquoteadv_Block_Adminhtml_Qquoteadv {

public function __construct()
{
  echo "works";
  die;  
    $this->_controller = 'adminhtml_qquoteadv';
    $this->_blockGroup = 'qquoteadv';
    $this->_headerText = Mage::helper('qquoteadv')->__('Quotations');
    $this->_addButtonLabel = Mage::helper('sales')->__('Create Neeeew Quote');

           ///////CUSTOM code for new button:
   $data = array(
           'label' =>  'Download Quotes',
           'onclick'   => "setLocation('".$this->getUrl('qquoteadv/adminhtml_qquoteadv/download')."')"
           );
   ///////The URL I am using is a custom module that I set up earlier, Magento parses it to <MySite.com/shop/index.php/downloadtomas>, which then runs the script I have in the IndexController.php file
   Mage_Adminhtml_Block_Widget_Container::addButton('download_to_mas', $data, 0, 100,  'header', 'header');
   ///////End CUSTOM code


    parent::__construct();
}

Does anyone know what I'm doing wrong here?
Thank you so much in advance.