Amplified speakers missing frequency range

[Kyle_M.]

i have an amplified system that was professionally installed (pre installed when purchased from owner)
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Deck-Stock premium cassette and CD radio
AMP-Alpine MRP F300
Speakers-(4) Polk db571 (possibly,cannot get exact model #)
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all speakers play sound without cracking or popping/going in and out.

Sound from iphone via cassette-aux or CD is missing part of the playback. not sure what section is missing. (FM radio seems unaffected but bass is weak)

Is there a way to test sections of playback?
Audiophile senses tingling
let me know what you need pictures of, Will deliver.

[GLH]

You have 1 of the best 4 channel amps ever made and Polk speakers aren't anything to spit at either. The weak link in your sound quality is the oem headunit.

[Kyle_M.]

anyway to beef my headunit without changing the face?
I like the stock look (aftermarkets are nice but tacky IMO)
had a very small gauge wire connect to the seat mount bolt but not connected to anything. any idea what its for/ missing from.

[GLH]

Set your tone settings on the headunit to 0 or flat or midscale (I can't remember how it works). Turn the volume on the headunit to about 2/3rds or halfway between half volume and 3/4ths volume. ( I normally say 3/4ths volume for aftermarket headunits, but oem will clip sooner). Since you do not have a subwoofer, be sure the filters or crossovers on the Alpine amp are off. That is where you need to start out and reset the amp gains. Start with the gain dials fully counterclockwise. You can use the DMM method or use a 1000 hz test tone recorded on a cd. You will need to fade fully front on the headunit and set the front gain, then fade fully rear and set the rear gain. The best of the 2 methods is with a DMM. You would use Ohm's Law. Use the speaker rms power handling which should be 60. Speaker impedance is 4 ohms. 4x60= 240. Sq.root of 240 = 15.4 volts AC at the amp outputs.