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. 

The gain is used with either type of input.  It is very possible that having the gain at the minimum setting is the best setting for the system matching.  It is also possible that it is too high even at the minimum setting.  You see, using high level input is actually a higher voltage input then the line level is.  One would assume that it must be better because it's higher voltage, but the bad news is that the signal level is noisier and weaker than line level.  Line level RCA inputs are always the better signal-to-noise ratio.

In setting gains, you will find that your deck will likely begin clipping at about the halfway point when you use the high level.  Use  the halfway point on the deck to set the amplifier gain (or use test tone sine waves to more easily locate clipping level).  You may find that the gain can be turned a little bit and the overall sound is better than what you have experienced so far.  If the amp's gain is matched to the deck while the deck's volume is at the halfway setting, that point will be the highest volume that you will ever turn the deck.

Generally, you would obtain best results by using a high quality aftermarket LOC rather than the amp's built-in high level inputs.  I would go that route if I were you.

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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.