Depends on the vehicle... All have different alts. My tahoe had 130 amp stock and held 2krms just fine... But then you could have a civic that has what, 80 - 90 amp alt?
What coils are the subs? Specs on the amp? it's probably because its on the cheaper side of amps. make sure his grounds are all good, and the amp isnt seeing a lower impedance that it should be