My experience is not massive with in car systems, but i like my music, my family is heavily musical and i used to do a bit of live sound engineering for my sister. I also have pitch perfect hearing so i notice bad sound and it frustrates me.
My previous installs most recent first are as follows: -
BMW E46 had a JVC head unit which was a single din DVD. Good sound, OK hands free good ipod control. This was hooked up to Infininity 6.5 inch components in the doors and rear shelf. I ran a very rare Power Acoustik area 51 amplifier. The amp was 4x200w RMS and was mounted in the spare wheel well as it's a 17" round amplifier. My local ice dealer sold it to me second hand. This ran the fronts and i had a directed 600W rms 12" subwoofer in the boot in an off the shelf box. I had the rear channels of the amp bridged to mono and used the head unit to power the rear speakers for fill. It sounded great, but was frankly too loud. Volume control went up to 100 on the head unit and my ears could only handle 15.
My classic car a BMW 635csi has an old system which comprises of a Pioneer Retro look head unit fitted in 2004. 3 Alpine V12 Amplifiers. One powering the 4x4x6" speakers. One powering 2x8" subs fitted under the rear seats and one mono amp powering a MTX 10" band pass box. It really needs better speakers, but i don't want to cut into the car, but have just found some 6.5" speakers made by Rockford Fosgate that have adapter plates for the 4x6 locations. This car has a hidden bass controller and it's the best part of the system. When i start my restoration on this i will redo the system using more modern components that were not available when i installed things before.
My first real car was a Pug 205. I had a top of the range Alpine Head unit with Infinity components front and 6x9's in a custom MDF shelf. There was no subwoofer, but tonally this was excellent.
Fist car was a 0.9 litre polo and had 9 speakers to make up for the lack of engine but sounded horrible as everything was cheap.
Thanks for the link to the shop. I have favourited this
Regarding the speakers i bought them from Audiovisual world see link.
I am not sure what is IB can you explain?
My stereo shop says the bass will be ok with power which i have, but suggests that if we need more we can do one of two things. Remove sound deadening in the parcel shelf or behind rear speaker covers drill 3" ports into the boot for the bass to get in.
I have not done anything underseat so would probable have them running of the head unit front channel for now, but if i feel the need for more then i think the earthquakes are the only way, but they are pricey. Then i would run the mids from one change on the amplifier and underseats off the other with the rear off the head unit.
as for the false floor i thought this was something i could do myself. How hard can it be. I am going to make out of wood two mock up amplifiers and then build the floor and cur the holes. This was i don't scratch my amps. The floor is basically flat. The it's just a case of carpeting with appropriate carpet. I was also thinking of lining the edge of the floor and the boot with velcro so that it does not rattle over bumps