Will 6.5" mids fit to original places if I use spacer? I'm thinking of DLS MS6. And subwoofer assembly to front also (Hertz HX200). Amp might be Xetec 4G300MKII, that might fit underneath passenger seat.

Don't know about the speakers, but there's a box under the passenger seat that takes up all the space. There's not much under the driver's seat either, as there's a bar that goes across under the seat which doesn't leave much space (something to do with the height adjustment, I think). I know, as they're the two places I tried fitting my CD changer...

I know that box, but can you remove it?

I know that box, but can you remove it?

Yes it's only held in by 4 screws. But if the amp is compact enough, it might be easier to put it in the box - cutting away some of the plastic if necessary to get the amp in. The bar across the seat frame is at least 50 mm from the back of the box, so there is more space under the seats than it looks.

Xetecs outer dimensions are 225 x 65 x 225 (mm). It's very compact. Xetec 4G300MKII

I fitted my Pioneer CD changer in the seat box, and that is 248 x 66 x 168, so with the back off the underseat box, your amp would probably fit. Picture in my gallery.

But will it get too hot down there?

That's the passengers problem ..."aaargghh, my *** is on fire!"