will option b work in this drawing?

I have a factory sub amp that I want to replace.

I want to do as little "damage" as possible, as it is an expensive car and I would put it back to stock before selling. 

My current setup has the OEM amp going to the stock 8" sub. 

I know I can tap the speaker level inputs prior to the amp and run these to the new amp, option "A". 

BUT, what would happen if I took the leads of the speaker itself, ran them into my speaker level inputs on my aftermarket amp, and powered down the OEM amp so as not to overload the aftermarket amp?  This would be unbelievable simple wiring, as access is all right there and where the new component will be located.

Would this work?  Or is there too much "stuff" to go through inside the OEM amp to get a clean signal on the other end?

I did my best to draw it out with MS Paint (I'm at work).  Thanks in advance!

If the fuse is removed from the factory amp, b will not work.  In diagram b, input signal for your aftermarket amplifier appears to be coming from the output of the factory amp.  No fuse = No output to supply to your aftermarket amp.

Most aftermarket amplifiers are not designed to accept input from any decently powered amplifier.  I would highly recommend looking into a PAC SNi-35.  This is a line out converter designed to work with a factory amplifier.

Copy all, thanks guys!  My B is a no-go.  Oh well.

IAI, I was trying to stay away from the LOC route.  My aftermarket amp is an ALPINE MRP-F240, and I believe it will accept the balanced/differential inputs from my OEM headunit (BMW).  There's no advantage to using the LOC, correct?  Tapping in front of the OEM amp (and disconnecting it from OEM sub) should work just as well?

Okay.  let me pick your brain...

What would be sufficient level to power/max-out an aftermarket amp?  How do I check it, crank it up pretty good and check the voltage prior to the first OEM amp, I would assume?

Don't LOCs introduce possible noise vs. going direct from head to amp?  I have always advised against using them to others if they were able to use the speaker-level inpuuts on their new amp...am I wrong?

If you are going to use a volt meter, you will need a cd with a test tone or pink noise.  If you are using an oscilloscope, you can use music.  Anything between 1/2 to 1 volt should be able to max out just about any automotive amplifier.   Yes turn the volume up to a decent level.  Check the voltage.

A line out converter does not add any distortion to your system, unless you are turning the volume up loud enough to clip the output of the Radio's amplifier section.  The Hi level input of an amplifier is simply a voltage divider that reduces the speaker level voltage to a preamp level signal.  The same exact process and the same exact way a line out converter does it.

If your speakers do not sound distorted to you, the signal going into your amp is the same quality as the sound going to your speakers now.  A Line Out Converter is simply a voltage divider circuit.  10 volts in 1 volt out.  There are no components in it that will introduce distortion into your system.