It's louder because it was
horribly recorded (or really added). The bass line on the chorus goes from about 36hz to 30hz. It's not the lowest ever and there are plenty of rap songs that get lower. The levels in that song are very ill proportioned and they are boosted over the other frequencies, thats why it seems louder.
http://www.caraudio.com/forum/showthread.php?t=172566
That thread has some decent tracks in it.
Ayreon - Day Three: Pain
http://fn4.putfile.com/getfile/1202092920video1234sslash34516230417.mp3 (right click on that to save it)
That's a great track that has some 30hz content in it as well. A lot of the Ayreon stuff has nice bass lines in it. The most recent album, 01011001, has some great bass lines in it but it's not really metal, more like progressive rock.
Lamb of God's Redneck and Laid to rest both have nice bass in them if I'm remembering them correctly. Good songs too. You also may like Bury Your Dead's Magnolia.