kicker amp with hi level speaker level

Hi, got a question, not sure if I got a bad amp or Im just setting it wrong.  The amp is a kicker zx650.4 which is used to power my 4 interior speakers.  I am using the hi-level inputs or as they call it speaker level inputs.  Now my question is, do I have to set the gain.  Cause the gain is set all the way down and the speakers play loud and increases nicely with the deck.   But as i increase the gain a little bit up I hear distortion and the speaker goes into protection. I think because the front speakers can't handle the power.  But honestly I heard the speaker play louder with my other amp.   So is the gain only used with the rca inputs. 

I have some corrections to STEVDART’s answer. RCA inputs are NOT superior to speaker level inputs and the signal to noise ratio IS dependant on where in the car you get it from. The big question here is dose your amp have a line level output converter (LOC) or a balanced differential input on board (both types will be referred to as speaker level inputs). Kicker support should be able to answer that question for you. The majority of amps on the market use differential inputs. The technical difference between these two types of inputs is complex. All you need from kicker tech is a definitive answer. If the amp has a line level out put converter on board and noise is not giving you problems then purchasing an outboard LOC will not do a thing for you. If your amp has a differential input on board and is not playing as loud as another amp in the same situation I suggest that you check your Hi Pass cross over settings If you hear distortion before you reach 80% volume on your head unit you should suspect that your head unit is over driving your amp. If you suspect that your head unit is over driving your amps inputs try inserting a voltage divider between your head unit and your amp. Place a ¼ watt 1K resistor between the positive and negative speaker outputs of your head unit and a ¼ watt 2k resistor between that positive joint and the positive output of your head unit. This will form a 2:1 ratio voltage divider and will cut the gain in half.