Setting gain on amp with multimeter issues with voltage? plz help

Archer3545 Enthusiast
Jan 3, 2020
TLDR: Today I was setting the gain on my with a multimeter, I calculated that to hit 50w rms I need to tune the amp to 14.1v I played a 200hrz sine wave and set both gains for the front speakers and the back speakers to 14.1v I changed the frequency and noticed that my front speakers seemed pretty normal and close to what I set them too when I was playing the 200hz, but my back speakers if the frequency was higher than 200hz the voltage would read way higher than what the front speakers would read and if the frequency was lower than 200hz the voltage would read would be way lower than the front. I tried tuning at different frequencies and nothing fixed the trend.

I have a stock head unit with the wires spliced to a lc7i LOC (which I have had for quite a few months now). I bought brand new
Rockford Fosgate R300X4 Prime 4-Channel Amplifier after my old skar one stopped working. I also replaced the door speakers which I had already replaced once, but one of the speakers were bad and the product was no longer sold. The new speakers are Infinity REF-6522EX Shallow-Mount 6-1/2 Inch Coaxial Car Speakers new.
Anyway I hooked them up and I was ready to set the gain on my amp the best way I know how. (With the lc7i you can leave the gain on the amp all the way down and then turn up the nob on the lc7i effectively still the gain, but a light will come up when you start clipping. However using this method was way too loud and I have had a couple of subs blown. I wanted to set it with a multimeter which I have done before a couple times and I know you do volts times the square root of rms watts times ohms. So I want 50w rms, with that equation I should be aiming at about 14.1v. Here's where the weird stuff starts happening. So its a 4ch 2ch bridgable amp, and I have it in 4ch mode. on my head unit I have all of the equalizer settings turned off. I have the hp and lp off on the amp and I have the nobs all the way down on my lc7i. I unload the speakers and hook up my multimeter and start playing a 200hz sine wave with my head unit volume at 25/30. I set both front and back speaker gains to about 14.1 volts. I play a different frequency, at 1000hz I check my voltage and on the front speakers, it read 14-15 volts, a bit higher than what I originally set it too. I check my back speakers and it says it's at 22volts. I check a lower frequency around 75 herts my front speakers were at 13-14 volts a bit under what I originally set it at and my back ones are at like 5-6 volts. A messed with the gains more I tried different frequencies. I tried tunning them at different frequencies and no matter what, frequencies higher than what I tuned it at the back speakers would have a way higher voltage than the front and frequencies lower than what I tuned it at the back speaker voltage would be way lower than the front. Does anyone have any ideas as to what may cause this? Bad amp, bad wiring anything I should check please.
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