Strange, someone else was recently complaining that their V70 had seats that were too hard... whereas I find my '98 S70 to be "just right" :)

I don't think there is much you can do to firm up the seats, unless they are softer than other cars of the same model (perhaps the support springs are not good?) in which case you may eb able to replace the seats with new ones. You could try out a '98 seta and see if it's more to your taste maybe they changd them later- and I have no idea if you can install the older seats into your car.

A cheaper and seasier solution may be to make/buy pads made of closed-cell foam or possibly "memory" foam. You could insert a layer under the upholstry to increase the firmness of the seat and lower back. Call an industrial upholsterer about that ior think about doing it yourself. Some companies specialize in refurbishing auto interiors they may be able to help.