Kicker C12 4ohm Sub newbie questions

Hello to all, i"m here about a 2 days, reading all post that regards to subwoofers
i"m planing to add a subwoofer to my bedroom ( about 3 x 4 meters ).
what i have for now:
2 x Wharfedale Athlantic 400SE floor speakers
1 x Kenwood KRF-V5100 Receiver/Amplifier ( 6ohm version ) connected to PC via SDPIF to Realtek on-board soundcard.

for now, i want to add a subwoofer to my system.
i searched the Israel market, and found thats its no subwoofer for HT, only the cars one,
i bought KICKER C12 4 ohm subwoofer driver. and start reading its tech spec, i found that kicker recommends 35-99 liters box for sealed. i dont know realy what this mean, what is recommended size for it ?
and second thing im stuck in selection between do a DIY sealed box, or go to store and buy car sealed or ported box (about a 35-40 liters ) ?

the price of boxes in store are from 60$ for sealed/ported one 35-40 liters.
the carpenter wants about a 70-80$ for this (17mm MDF ) job excluding the assembling the box.


if i select to DIY box i decided to do a stealth box under my bed ( 200cm x 160 cm ) size of box whill be something like 18cm ( free space from top of underbed to floor -2cm ) x 60cm x 65cm. this will fit nice under my bed, but i dont know if this sound good and give me a good bass ?

3th thing that i don't know how to connect it to my amplifier in good way ?

1. connect it in parallel of the one (left or right channel )
2. connect it in series to one of the channel
3. buy Noname car 4 bridgeable channel amplifier power it from dedicated 400w PC power supply and connect it to subwoofer line out in kenwood amplifier ( pass thought, not any cut offs only mix lows from channels).

i think the "3" give a more options ( crossover, phase )

my prefers are music (70%, movies (20%) , games ( 10% )

sorry for my English i"m Russian from Israel

waiting for all answers.