2016~2023 Finally replaced the stock CX-9 Bose subwoofers

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22 CX-9 GT
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21 6 Carbon
I had one of my Bose door subs blow, voice coil fell apart, so I decided to replace it, as well as the other door and spare tire sub. The stock Bose door subs are 1 ohm and are very expensive to replace. They actually sound pretty good, after some door treatment.

My plan was to add a 4 channel amp to power the new door subs and add a new 12" subwoofer. I wanted less bass in the doors and more bass in the back. I wanted to keep the spare tire and have it be accessible. I decided to place a 4 channel amp inside the factory spare tire and then build a sub box to take the place of the center rear storage compartment. I built some speaker adapters for the doors and wired them up.

First I installed the amp. I choose the Pioneer GM-D8704. The + battery terminal on my Grand Touring had 2 open spots on it, one fused and the other not fused. I decided to use them both. The unfused port received an additional 4 guage power connection to my alternator. I connected 4 guage wire to the fused connection, ran it to another fuse and then to my amp. Grey covered cable is ran to alternator. Ran through factory rubber grommet and down driver side to rear.
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I also upgraded the factory engine compartment grounds.
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Next I built the 12" sub box. I used the rear center storage tray as a template and calculated that it needed to be a max of 4" high. I used an Infinity REF1200S as it had shallow mounting depth as well as good quality T/S parameters. It is rated for 250 watts RMS, I am feeding it the max my amp will do clean, 333 watts RMS. The box came out to about .8cuft. This is the bottom of the box, it's down firing.
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I then built the speaker adapters for the door subs. I choose the Peerless SDS-135F25CP02-04 5-1/4. It is a small subwoofer that can play to about 1200Hz. The stock Bose door subs have input signals from 30Hz to around 600Hz. the Peerless are rated at 50 watts RMS but I crossed them 12 dB high pass at 40Hz and will feed them 100 watts RMS.
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Next was time to wire everything up. This is the factory wiring diagram I used to find the speaker colors.
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I found that the factory Bose spare tire sub had a differential low level signal going to it from the factory amp under the passenger seat. I completely cut it as I used all 5 wires from it. It has a constant 12v, ground, 12v remote turn on and 2 differential wires. My Pioneer amp accepted low level or high level inputs but not both so I had to put in a line output converter. I also wanted a remote level controller for the 12" subwoofer so I bought a line driver with one. I decided to use the Wāvtech linkDQ with linkRC for the 12" sub and link2 for the door subs. These are very high quality devices. I used the Bose spare tire sub harness to power, ground and turn on these 2 components.
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I found the factory door sub speaker wires and cut them by the kick panels. I ran speaker wire from that point to my link2 and then from my amp back to where I cut them.
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I spliced the factory Bose spare tire sub signal wires into my linkDQ and ran it into my amp. I ran ethernet from the linkDQ to a spot under my steering wheel, between the door and wheel. This is where I placed the remote for the 12" sub. The linkRC is able to be completely taken apart so all I have showing is a very small knob.
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I then added the sub to the back.
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Finally it was time to tune everything. The factory EQ curve is not flat. There is a huge bump at 60Hz for the spare tire sub signal and high pass filter around 30Hz. I 12dB low pass crossed it on my amp at 40Hz to tame the factory 60Hz bump. I set 40 as my max volume level. The bass between 35Hz to 55Hz is overwhelming during certain kinds of music so the remote is very useful.

Stock EQ curve voltage. Voltage out of Pioneer amp. 40Hz high pass.
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I high passed the door subs 12dB at 40Hz. I have before voltages and there is another bump at 60Hz and a huge bump at 130Hz. I had to use 125Hz to tune the amp as that was the closet freq I had to 130Hz that was -5dB. The door subs play from 30Hz to around 600Hz. This is the curve after the high pass applied.
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Overall the system sounds much better. The door speakers now have great midbass and are much cleaner. The 12" subwoofer is much more powerful and musical now. I used my multimeter and cheap oscilloscope to test everything and have lots more data and pictures. I played around with boost at 30Hz but prefer cutting at 40Hz instead. I'll try to add more info later.

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Last edited:
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22 CX-9 GT
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21 6 Carbon
I just want to add the pin out of the stock subwoofer harness. I initially had the low level signal wires reversed polarity.

The colors are as follows:
Black - Ground
Red/Light Blue - Constant power
Green - Remote turn on power
Red - negative differential signal
White - positive differential signal

I initially assumed that red was + and white was -. To test, I set up my factory sub with my inverter and connected a 9v battery and saw that it was the opposite of what I thought. I then used my polarity tester app and verified it again. I also saw in some other posts for the Mazda3 and 6 that follows the pattern of the differential signal cables from the tuner to the amp. It appears that on connector 0920-319A, (O, M, K, I, A are positive) and (P, N, L, J, B) are negative. This was confirmed by a Mazda 6 post where it was tested with a multichannel oscilloscope.

Correct polarity.
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Incorrect/reverse polarity.
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Tuner to Bose amp.
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Bose amp to bass box.
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This is a Mazda 6 pin out, tested with a multi channel oscilloscope, from a different forum.
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