Clipping.

[k58.cross 15]

Like others said, the clipping happens in the amplifier. I don't think you should have traded that stronger lvl5 for a weaker lvl3. But anyway, about the clipping...

You don't have much current supply, so to compensate you've probably had to increase your source volume and/or gain too high. Both of those things can cause the amp to clip. For now, back off the gain until your clipping goes away (at the highest volume you play your music). To get more output, you need to increase your current supply. Both a high output alternator or extra battery can do that for you, but putting in an extra battery is the cheaper and easier way to do it. Research these topics yourself so you can learn the details and get an idea of what size they should be. 

You also should put in a dedicated voltage monitor. Watch the voltage at your battery terminals and make sure it's not getting too low. First the voltage will start at 14+ after turning the car on, which means your alternator is supplying most of the current. When the alternator can't supply any more current, voltage will drop to around 12.8 and your batteries are now supplying most of the current. The more you use them, the farther the voltage will drop. If they get below 11, charge them yourself manually (need to buy a charger). The alternator will provide some charge to the batteries when music isn't playing, but for best health you should charge them manually every so often.

 

EDIT - Well I missed the part about you having four batteries before and still having clipping...Your gain or signal volume had to be too high. If they weren't too high, then the amp is suspect.

Edited February 11 by k58.cross

[strangeduck 4662 1 Vehicles]

turn the gain down on the amp as low as it will go, turn the volume on headunit up and see if it sill clips at 50% volume