Since SPL target mo i would recommend DD 512, 7,400 price nito. 250 watts rms lang to but you can power it to 400 to 500 watts para mabusog. Tapos sa box phatbox or abc para bayo talaga.
Yung Focal seps na lang kunin mo mas okay yun kaysa mag coaxial ka.
With regards to your lola's heart, sa HU naman ma ooff mo yung sub kung sakay mo lola mo. This is what I do pag kasama ko parents ko sa kotse, usually off yung sub or mahina lang yung setting ng sub