The two songs I use:
Daz Dillinger - My System
Billy Joel - River of Dreams
The first one just has low and loud bass throughout the whole song. The words are pretty worthless. The second song has a very low, very quick bass line throughout the song. Most subs will make it sound like one big blur, mixing all the notes together. Your better subs will allow you to distinguish the notes.
And in case you care, I have a Brahma 12
If you're into the metal stuff, Fear Factory - Obsolete is a pretty good album on subs. I wouldn't reccomend running 15's cuz the guy's double-kick is like a machine gun and your sub will be sloppy as hell.
No it won't. If it comes from a crappy company that uses the same motor on their 15 as they do on their 8, then yes, it will be. But a 15 from any of your reputable companies will sound fine.