I still have my 2001 factory HK HU installed and I figured I'd easily install an aftermarket amp and sub. SO I just bought a low line level converter and tapped into the wires from the back of the head unit (I think it is those and not the amplified comming from the factory main amp), and from this low line converter, I am running the rca's to the MTX amp using a very decent set of rockford fosgate cables. Anyhow, even with my old MTX amp, I have always had an issue ot my amp cutting in and out. I know it is not the power, there is constant power, and it doesnt seem to be a remote wire issue, cause even if the bass is not very noticible, there still seems to be some rumblings at a low volume comming from the sub. I also tested my wires that I hooked up from the HU wires, that go into the Low line level converter with an ohm meter and they seem to be fine. Could it just be that MTX amps dont work well with BMW wiring? Or something with the way BMW factory HU's put out their signals from the head unit to the factory amp? BMW's do NOT have hu RCA pre-outs right?
If it is an amp issue, of not being able to handle the input signal, what are other good amps that are cheap that could solve this issue?
Clockwork

Poor amp ground? Improper impedance presented to amp due to improper voicecoil configuration? Does it happen at low volume, high volume, is the amp very hot when it shuts down? ETC... ETC... ETC...

Poor amp ground? Improper impedance presented to amp due to improper voicecoil configuration? Does it happen at low volume, high volume, is the amp very hot when it shuts down? ETC... ETC... ETC...

ditto, we need more info.

I vote bad ground.

it's not an mtx v.s bmw problem. both my amps and my subs are mtx and they work fine.

i could be wrong cause i don't know about the hk unit but i think the signal coming from the stock Hu to the factory amp is low level. you don't need the converter.

when i installed mine i took the rca wires, cut the plugs off of one end and spliced them to the wires going into the factory amp from the head unit. splice them accordingly, the insulated wire in the rca's is positive. then i pluged the other end of the rca's into the 4 channel mtx amp. i used the output on the 4 channel to get the signal to the mono amp. tune it and have fun. it sounds great.

you can do what you did but you'll have to get the signal from the factory amp. it might sound like ass too.

my MTX amp is capable of taking speaker wire into it, not just RCA's so I could do what you did, but instead of wiring an RCA cable in, I could just run some tapped wires from the HU wiring (before it gets to the factory amp) and plug it into the special inlet on hte MTX amp. I just heard that this methos results in shitty sound quality.
I was also told that BMW uses a special type of wiring method to transfer the cound ffrom the HU to the factory amp, and that is why they dont use simple RCA's to run fromt he HU to hte Factory amp. I wonder if this special wiring technique is some of the problem? and I was told a "peripheral, Super Vendetta 2" will solve my sound cutting in and out problem. somethig to do with teh sound being a balanced signal, what ever that means. any ideas on that?
Clockwork

no special wiring, in fact the factory amp is the same as what you would be doing using the high inputs on your amp.

The stock heads create speaker driving power (no amp needed, look at 318's) and the amp actually has a high level input.

shitty sound quality? no, not really. comparitively that is.