TL/DR: 02WS6 did his math wrong. The dual 4 sub will receive 2/3's total power and the dual 2 sub will receive 1/3 total power.
Another TL/DR for most but feel free to read for some entertainment value:
How? You explained how to wire the subs, not how to change the internal impedance of the voice coils which is impossible. Resistance or load the amp sees ≠ Impedance
You're missing the point but I'll do my best to help you refocus on the actual problem OP is facing if he followed your advice.
Again all you did was tell him how to wire the subs together to provide a complete resistance load to the Amp of 1.3 ohms. The amp should be fine, no one in this thread contested that. I’m contesting how the load is split between the two subs.
Do you know how forums work or just Facebook and Reddit? Forums have post counts and join dates for a reason. Those with higher post counts and who have been members for a longer period of time USUALLY, have seen more crap and are more experienced. I’m speaking in generalizations, I’m sure there are plenty of senior members who are wrong from time to time but I have a feeling they’re wrong less than someone with <50 posts when it comes to car audio. Seeing as this is a car audio forum those post counts mean the more "Senior Members" have contributed a helluva lot more to this place than either of us have and I for one respect that, much in the same way I'd respect it in any other forum. If you don't like it then go hang out on 4Chan, they hate post counts or usernames over there.
I'm not contesting your original response, only that you glossed over one of the main issues of wiring two different impedance subwoofers together:
Correct on all counts - Again my issue is you glossing over the fact that the two wildly different subwoofers don’t get the same amount of power. Just to recap in case anyone forgot
Shallow Mount - D2 - 400W RMS
vs
Standard Mount - D4 - 1200W RMS
I have nothing to contest here, it's all electrically sound except for one formula you forgot to mention. Which you even mentioned above and I highlighted for good measure. "The subs wont get the same amount of power". You're exactly right but then you're not even remembering that you brought it up! How does a current load balance in a perfectly imbalanced circuit? Let's say for example, 2 ohm voice coils vs 4 ohm voice coils? Oh that's right it still follows the path of least resistance so the 2-ohm voice coils will be getting about 70% of the current where the 4-ohm voice coils will be getting about 30%. No matter how you wire the final load the amp sees, you still cannot change how the internal voice coil impedance will consume that power. You've protected the amp, but the lower rated and more sensitive sub will still be getting 2/3rds of the power.
The remaining 30% of the power the amp is producing, is consumed on a sub with a lower sensitivity and a much higher RMS rating. So as OP turns the gain (or volume) up, the P3S will be dramatically louder than the Gothic, if he continues turning the gain to get anything out of the Gothic, 70% of that power will keep going to the P3S until one of two things happens. OP realizes that the P3S will blow before he gets anything worthwhile out of the Gothic or he blows up the P3S trying to get the Gothic to hit. THAT is the gear we all believe he's gonna blow up if he wires it the way you advised, the P3S Sub NOT the amp.
Actually load the amp sees is exactly what impedance is. Load the amp sees is better described as impedance than resistance. We just generally use the term "resistance" because it's easier for people with electronics knowledge to understand and it's much easier to measure/test for. Impedance is a combination of resistance and inductive reactance, and has to be measured by Z = E/I.
The points raised by the elite, all-knowing, super, high-post-count guys haven't mentioned blowing subs. The conversation ranged from a generic "Don't do it. It's bad", to cancellation from running two different subs. Having said that, you are absolutely wrong with your 70-30 statements. Please give me the formula I forgot to mention that you alluded to. Although I didn't mention power levels, because I assumed OP can set his gain levels to avoid blowing either sub, let's break it down so you can learn something (even though I'm a moron because I have under 50 posts):
This will apply to a series-parallel configuration with two 2 ohm loads wired in series, and two 4 ohm loads in parallel with the series 2 ohm loads, so essentially three 4 ohm loads in parallel. I'm not going to use numbers because this doesn't apply to only one scenario, so variables are more appropriate. I'm going to refer to the voice coils as vc2-1, vc2-2, vc4-1, and vc4-2. Vc2-1 and vc2-2 are the voice coils on the dual 2 ohm sub, and vc4-1 and vc4-2 are the voice coils on the dual 4 ohm sub. You should be familiar with the rest of the formula symbols. On to it.
In every parallel circuit, supply voltage is the same across each branch. Vc4-1 is its own branch; vc4-2 is its own branch; vc2-1 and vc2-2 in series represent a single branch.
Therefore, Evc4-1 = Evc4-2 = (Evc2-1 + Evc2-2), or to rephrase, Evc2-1 = Evc2-2 = 1/2(Ecv4-1) = 1-2(Evc4-2)
Current is limited by resistance, and can be different between parallel branches, but is constant in series loads.
Given that I = E/R, and each parallel branch is equal to a 4 ohm load, Ivc4-1 = Ivc4-2 = Ivc2-1 = Ivc2-2
The formula for power is P = I x E.
So, since Pvc4-1 = Pvc4-2 = (Pvc2-1 + Pvc2-2), we can see that power is cut into three, with the dual 4 ohm sub receiving two thirds of total power, and the dual 2 ohm sub receiving one third of total power.
If you want to simplify it, the dual 4's in parallel are a 2 ohm load, and the dual 2's in series are a 4 ohm load.
I think we can all agree that a 2 ohm load draws more power than a 4 ohm load. Roughly twice as much.
I hope that makes sense to you. You can let me know if you have any questions, but you'd probably trust someone with more posts more than you'd trust me.
- Joe