That's because you where still using a low quality budget amp then replaced it with a slightly better low quality budget amp. Imagine what it would have sounded like if you had bought quality like brax, zed, rf power series, jl slash, or Zapco. I have always heard a difference in sound quality from one amp to the other. Me and my friends all ran 12w6's and infinity kappa's in high school and they always cheaped out on power and couldn't figure out why my M100 and M50 sounded so much better. Maybe it was just more power but it deffinetly made a difference. Now if I just had a sub amp I probably wouldn't have even noticed but it was the entire system that was better. Lower noise floor, and a lot more accurate and clean sounding, not to mention a direct swap resulted in gaining 1/12 db's. It actually amazing the difference in a budget install with budget components and a high end system. I can tell just going from my dodge to the mustang how much a difference quality makes. Even at low levels the mustang will destroy your ears where the dodge can be blasted at full volume and want bother your ears or sound harsh. But one is a spl install meant to be loud and the other a sq install so that could be part of it.