I would not suggest using rear speakers if SQ is your goal. It can be done, just way more work.

Larger subs would be more beneficial in a larger car. So if your car is big go with a big sub or multiple smaller diameter subs. The focal subs are very good. But you could get as much or more performance from less expensive products. When I purchased the speakers for my car I based my decisions on each drivers singular performance. I left the brand name at the door. I don't get paid to advertise so why would I care what the brand name was. Your install can make even the least likely products blend in with the car.

With little to no budget in mind, my personal favorite sub would be the JBL 15WGTi. It does everything well. But its pretty big. I ended up using the Alpine type X in my car as it had almost all the performance of the GTi and was much less money at the time. The ID Max is a good choice as are many other products. Peerless make a great SQ sub that is actually affordable.

Get it installed right and you can't go wrong with any of the choices mentioned.

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"I have a 1500 watt hair dryer, but it sounds like sh!t"