I also have the the D30 case and have never had issue with echo??
I have also done this hardware mod here:
http://xdaforums.com/showthread.php?t=1441510
Its actually really simple.. only have to remove like 6 screws and pop the back off... just make sure u dont block the sensor from making contact with the back plate..
and if u attempt this and break ur device... NOT MY FAULT!!
i have the impact D30 case and have to echo whats so ever ......