Amps draw a lot of power, this is the reason many people with good amps in their car have a 2nd battery, or a large capacitor in-line with the amp, both for preventing wire burnup with big bass drops that suck power, and to mitigate the 'drain' when the amp is in use. You may be fine for a week or two, but then all of a sudden the car wont start, because you'll have sucked the battery dry.
This actually happened to my TR6. I didn't know my alternator was going bad, the voltage looked fine, but my 8inch powered sub and loud stereo was draining enough that eventually i got stranded at work, and had to get jumped to get home.