Ok, so I know I'm new here, but I feel I can contribute with a cheap and easy mod you can do to both the MPS3 and MPS6. I wasn't the person who thought of this or the designer, but I do know a little about electronics and I can explain how it works.
With the standard Bose stereo, you will find that to get some decent bass you need to give turn the bass setting up on your dash to + 2 or 3. Unfortunately, when you do this, it limits how loud you can get your system because your door speakers start to distort when the volume is in the low 20's. However, the subwoofer is not being used to it's full potential. This little mod turns the volume up on the sub so you can decrease the overall bass setting on your dash which does 2 things. 1, Allows you to turn the stereo up louder, giver you stronger treble with less bass from the doors. 2, At lower volumes, the midrange is reduced giving the bass a 'punchier' sound.
The circuit is simple, only 5 components and can be built by anyone who can solder really. I have a wiring diagram, I cant take the credit for it, I didn't draw it. I have attached it below.
I bought all the components from Jaycar, you will need,
1 15k resistor
1 4.7k resistor
1 100uF Capacitor
1 1uF Capacitor
1 LM741 OpAmp
1 Small solder board to build it all on.
1 standard spade terminal.
Once you have built the circuit you are ready to put it into your car. It is wired in just before the sub amplifier. Open your boot and have a look at the sub mounted in the parcel shelf. The black box underneath it is the amplifier. Unclip the lead going to it, and undo the 1/4" bolts holding it to the sub, carefully lower it down and disconnect the red and green wires to the sub. Be very careful, the sub terminals are very fragile and will break easily, dont let the amp dangle from the sub. Once you have it removed the amp, you can install the circuit. The wiring is as follows. Connect the +12V source to the orange cable on the amp (splice it in). The ground source is the black wire (splice it in aswell). Splice the -Signal wire into the yellow wire. The purple wire needs to be cut, once cut, the wire that comes from the plug is your +Signal source, the wire that goes into the amp it your Signal output.
When you have connected everything up, wrap it all up in electrical tape neatly and find a good spot to stuff the circuit. The circuit inverts the signal coming from the dash, so you need to swap the wires onto the sub. you will need to replace one of spade terminals so that you can swap the wires, one is bigger than the other.
Connect it all up and bolt it back in. Get in your car and test out possibly the best stereo mod I have ever seen. If I have missed anything or you have any questions, just ask. Also, if people aren't confident with this sort of stuff, I would be willing to build and/or install these for people for a small fee.