weird sub noise

First, I know this a big topic but I cannot find a post specific to my issue. Here's what I'm working with: 1995 Ford Explorer (190,000 miles decently old), (HU) an Alpine CDA-9884 (ordered offline), (subs) two Kicker Solar-baric L7 Dual 4-ohm, (amp) 2500 watt Mark Antony, brand new RCA monster cables and a capacitor. I bought the Subs and AMP from a friend, I listened to the subs in his car and they sounded Great (very great). He helped me do the install (RCA on opposite side of vehicle from the amp power). Here's my issue: With the volume all the way down on the HU you can hear the subs hit (boom) if you change tracks on a cd/ if you rev engine it hits the engine/ sometimes my blinker will make it hit. When i try to use my subs at a louder volume the sound is very bad not even making a solid hit but goes in and out and all-around is a horrible sound. My subs still hit with music but with the sound coming in, the bass is no Bueno! (spanish). My ground for my amp is good to the best of my knowledge (have not tried moving it yet). I purchased a "Ground Loop Isolator" to see if that would help but when I plug up the ground loop isolator my subs do not work at all. I brought the part back and they tested the part at the shop and the part works fine (shop owner said it was my amp 100% and advised I buy a 4 million dollar amp). I'm not sure if its the amp because it worked fine in the old car. Ive seen similar issue on forums where it can either be my HU, AMP, Ground, Or even my Vehicle. Ive has three different sets of RCA cables going from my HU to AMP and all 3 had the same noise. My HU was ordered offline (which may be the issue). But trying to see if anyone can help, Any information would be greatly appreciated.

This problem is usually due to your sub amp gain too high and/or a bad amp ground.  Read and follow the "what is a good ground" and the "how to set your gain" sticky posts on this forum.

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Yeah, I've put the gain down pretty low and still noise, It may be the ground I will read those topic, thank you, still any more info would be good if anybody else has ideas.

Also, any ideas why the Ground Loop Isolator made the subs start working all together??

*Stop* Sorry =(