Has anybody been able to install a sub under the backseats of the S40 and what size? Im thinking maybe one under the left and right. And, by chance, will this not void the warranty?

 

If you think it might void the warranty it probably will. Especially when it concerns Volvo. I would do anything till your warranty is up and or you have loads of money to pay if they void your warranty.

And I am not an expert with the s40 series at all. There is room under the backseats for subs?

 

Even if there is room under the seats for subs...the accoustics would suffer a great deal