So would you say a 10" is plenty for a car (considering I don't really go for the whole boom boom thing, but just like a full rounded sound) but open up its top end?

I did have a 10" for a while (loaned from a mate) and I had it just hooked across the rear outs (amp was set up to do that) so there was no crossover at all, but when I changed to my new Impreza, all I took was the amp (4 channel 760W) so I have stock speakers and no sub. Vaguely looking to get one, but it is lower priority than various other purchases.