Retrofit Subwoofer on High Performance w/o Subwoofer

[IMAY2023]

Hello, I am going to make this change also I found a Volvo box for $220.
On the Harman Kardon or B&W Option, are they the same subwoofers?

Last question, can you hear the subwoofer well?

Well. it depends on the model. 
First of all fittings are different between the models and the location (right or left) . However, some people here made it fit by drilling new wholes or bending the sub's frame. 
Second, S90 B&W subwoofer is 8 ohm but HK SUB on s60 is 4 ohm. I don't know what is the impedance of s90 HK SUB.  So, here you need to be careful. 
If you use B&W AUD from s90 which has 8ohm minimum output, you must increase the impedance of the SUB if your SUB is 4 ohm by putting 4 ohm high power (50W) resistor on the + wire. Otherwise, you can burn the output channel of the minimum 8ohm B&W AUD.