The GTV has oval speakers in the doors, slightly smaller than a 5 x 7.
Most people use an adaptor and install a 13cm in there, you could use either a co-ax or component set depending on budget.
If you want something to really kick ass and sound top with very good bass I would recommend a set of Focal 130 V2, but they will have to be installed properly and use some Dynamat also, makes a huge difference.
FAO DevonDan. You should go for the new JL Audio 6.5" W3 sub, Awsome. Those other ones you are looking at are more a mid bass than a woofer.
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