ok guys a little update, as I stated this will be done over time as I am in no rush BUT want to make sure I get this right for the best sound.
SwissXC90, thank you for the above information with out this I would have struggled a lot.
So after doing some research and talking to audio Tec I think the best amp out there is the Pioneer D1004. It is a very small amp so I might stand a better chance of installing somewhere in the boot pannel with it being out of sight.
Also as the Volvo sub is a 8 ohm speaker (in total) with 4 ohms per coil this amp will support a bridge at 90w (true watt) per coil at 4ohms, more than enough to get good bass sound.
link to amp is here:
https://www.dynamicsounds.co.uk/pion...fier-400w.html
will update more as time goes on people.
Ryan.