Were you able to get the sub completely hidden? I had no Idea there was so much space under the dash and really keen to see photos if its possible? I presume you hid the amp under there as well.I am not a big fan of the Alpine KTP 445 power pack. I have that unit in my other car, and it is barely louder than the built in amp on my Alpine CDE 125BT head unit. I went with a relatively cheap Cerwin Vega Bomber (B54) amplifier that is really compact, almost as small as the Alpine unit. It works easiest with RCA inputs, but the factory Alpine units have RCA outs, so that is pretty easy. The only real challenge is getting a decent power wire, but I just ran a new 8 gauge from the battery to the big grommet in the front of the car. I would not mess with going for 6.5" speakers (I did it and went back to stock speakers for now). It takes quite a bit of work to modify the door panels to get a 6.5" driver to not hit the door card. A 5.25" should be okay, a 4" definitely would be fine, even with a large excursion. The speakers definitely want more oomf from the amp. Focal 5.25" should fit into a hollowed out factory speaker mount and not cause any problems, or you could order new Focal 4" Coax speakers (our tweeter locations are only about 6-8" higher than the mid driver location), and get new housings if you want to keep your factor speakers for future use or whatever. There is a matching aftermarket speaker adapter that matches the factory adapter shape and mount pattern if you search Ebay for '2005 Fiat Punto speaker adapter' and they are about $20. I went with the B54 amp, new head unit (that has a video in for a backup camera), and a 8" shallow mount sub in a 12"x9"x6.5" box mounted up under the dash. I wll probably go with either the Focal or Infinity reference series 4" coax speakers since I am getting plenty of bass from the sub.
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