Let me start by indicating the stuff im using.

Stock HU
4 ch Alpine MRV F545 125 rms x 4(for the rear 6x9 & the briged for the sub 500 rms)
2 ch Fosgate 250/ 62.5 rms x 2(for the fronts/ i like the way they sounded and plus independent treble and bass feature)
Replaced the 15" JL w6 with 12" MTX 8500.

Well, everything sounded about the same except the bass sounded much tighter, punchier, deeper, i mean these MTX 8500 will blow the old schooll JL anyday except for their efficiency. The JL are more effecient despite the similar DB sensitivity/ probaly due to 3 ohm load vs 4 ohm load.
Anyways, i have noticed after replacing the sub, the bass is weaker at high volume (probaly due to the amp since they are rated at 500 rms at 14.4 and i was testing at idle), and the highs gets diminish at high volume. What i mean is, the highs at high volume sounded like someone is reducing the treeble on the HU, is really weird, and if i turned the volume down , it will get back to normal sounding. What might be wrong here?

Any input is apreciated.

 

You mean aside from all that stuff you way that is erroneous and irrelevant?

I suspect that if you pull the fuse from the Alpine amp and retest, the treble problem goes away.

If you are showing the bridged alpine 3 ohms instead of 4, it is pulling a LOT more current, and there's probably not enough to go around.

The Alpine is NOT 3 ohms bridged stable.

 

Acutally, i should add, that the alpines are powering the rears 6x9, and what i noticed about the treble problem is from the front which is powered by the rockford fosgate. So what do u mean by pulling the fuse?

By the way, the 3ohm load is from the JL since they are dual 6 ohm wired in parelled. Right now i am running a single voice coild sub.

Thanks.