ok thanks or help if i just join the speackers to the back speackers as this is were i had my old 6x9s they work fine just not very loud as head unit only 50 watts so do ya think the amp is knackerd as mate who got it off had probs with with his sub using this amp but thort was the sub that had gone not the amp
Probably nothing wrong with the amp, you may have a power supply problem or a mis-match on the input side.
If you check the voltage at the feed point on the amp when it’s on but not playing it should be no less than 13volts.
Switch it on and play it at low volume and check the voltage again, if it drops more than a couple of volts, chances are the cables and connections are inadequate or the cars battery is knackered.
The more you turn it up, the worse the situation becomes. What you are hearing is distortion caused by an amp running under voltage, not overpowering the speakers.
Forget square waves and the like, you wouldn’t have the equipment to measure them anyway.
You have proved the speakers work, now check the amp supply.
Check the input requirement for the amp and the output from the H/U against the makers spec. (I am not talking about speaker levels, but the line levels, should be something less than 3volts output from the set.) a big difference will cause the problem.