Loud thumping with gain turnt up

Well a friend of mine just recently gave me his old amp. A Legacy LA790, he had it pushing 2 MTX 12's and it sounded fine. I am currently running 2 Audiobahn 1251T's for now and the amp I ordered (Audiobahn A8000T) was out of stock and wouldn't come in for 3 week's. He gave me his Legacy as a temp amp to run my 12's. Well I hooked everything up, good ground, I seperated the RCA's from the Power cable. Ran them both on Seperate side's of the car. The friend claimed he knew how to set the amp up so I let him adjust everything, needless to say he didn't know as much as he claimed because he ran the amp in Full Range instead of Low Pass. So I readjusted it to run in Low Pass but I noticed if I turnt the Amp up past around 30-40% power I could hear a thumping sound with the volume turnt all the way down to 0. I started messing around with the setting's and turnt the bass boost up slightly and it was a rapid pulsing sound that sent the amp straight into protect mode. I thought maybe it was a bad ground or maybe I had the RCA's ran to close to the power cable so I re-ran all of the wiring and seperated them as much as possible. But in the end it was still making this loud thumping sound. I decided to run it in Full Range again which gave me full control of the gain and bass boost but the thumping was still there, it just didn't go into protect mode when I would turn the gains and bass boost up like it did in low pass. I reversed all the wires, bridged the amp (both speakers bridged sent it into protect, single speaker bridged still had the same thumping). I still have a good 2 week's before my amp will be here so I was wondering if any of you guy's had any idea what might be causing this? Any help is greatly appreciated. Thanks!

You have a proper matchup of equipment there, Audiobahn and Legacy.  Just keep it on low pass, adjust the frequency to below 100 Hz, turn the bass boost all the way off and leave it that way, and turn the gain down to the point where the thumping stops.  There, that's all there is to it.  But also check the sub wiring and refer to http://www.sounddomain.com/shop/wooferwizard.pl to make sure the impedance isn't too low.

The thumping never stop's. The lower I adjust the gain the quiter the thump is. I've had it turnt nearly all the way down and you couldn't hear the thump and the speaker's weren't moving but it was set so low that my subwoofer's basically made no sound what so ever. I just have it set now to where you can't hear the thump unless you open the trunk.

Next step is to start over from scratch...and leave your friend out of the picture.  Crutchfield and others have good web step-by-step instructions for setting up a system.  Verify the sub wiring impedance is not too low for the amp.  Make sure all power and ground cables are at least 8 gauge, and ground points are sanded free of paint.  Read all manuals and understand what they say.  And always disconnect battery negative before removing or replacing any power or ground wires.

Alright, I believe my box may have been wired up wrong. I went by the diagram on SoundDomain and just rewired it but won't know for sure how it will work out until tomorrow since it's 12am almost. Don't think the neighbor's would take to kindly to alot of loud booming this late well early! So you know I am running a 4 gauge power and ground and 16 gauge speaker wires. Also one last questions. I plan to run a mono amp soon, I was planning on going with the Audiobahn 8000T, but I've been reading about how good this Vibe 1200D is supposed to be. Any helpful suggestions on what type of mono amp to run. Thanks for all the help!

Well, I hope you find the problem.  As you can see, one little mistake in wiring will make a drastic difference.  As for brands, spend a little time in this forum....read posts on the first few pages here and you'll quickly find several amp brands that are worth your investment of time and money, and opinions given.  Good luck.

I would not go with the vibe 1200d, it is not 1 ohm stabel like it says pluse I have installed one for a colledge kid and it lasted about 2 months, had to send this amp back 4 times, I only to have it blow up again and it runs really hot. I would go and check out the kenwood excelon KAC-X811d, I personally have had this amp since it came out and have never had any problems out of it, i have run it with a 1 ohm load for a long time to a pl3 and hitting 135db with just 1 12" pl3 setup. you will have to go with  at least 2 guage wire though but well worht it. got o locustaudio.com and get it for 235.00. I sell the same amp for 499.00, This amp will do 1000w at 1 ohm and 900 at 2 ohm no problem.

Well, rewiring the box helped somewhat, I can actually have the gain turnt up around 30% and the thumping is completely gone and the speaker's don't move like they were doing. But I still can't turn it up over 35-40% gain. I ran the speaker's to 1 terminal and bridged it on the amp, should I leave it bridged or run it normal? I am starting to think it's just the amp is going bad but I don't know. And amp wise, I have been looking at a Kenwood KAC-7201 and a Power Acoustik LT 980/2. Do you know anything about those 2 amp's? If I was to go with Mono it would probably be the Audiobahn A8000T unless you guy's know something I don't.

Once you understand what a gain is you won't be talking about percentages.  Go back to my first response and re-read it.  For more info on gains, start with a search on this forum, and you could find as much as you want also using a google search.

Well after a day's worth of fiddling around with the Low pass, I found the highest I can run the amp without the thumping, but the only problem is the speaker's sound like well crap. You can barely hear the sub's and they are barely even moving. I adjusted the frequency and that seemed to not help it. I don't really know what else to do besides just unhook it all until I get the new amp and hope the sound doesn't stick around. But again thanks for all the help.