Good Crisp Audio in a RRS - what do i need?

[L888DER]

Recently purchased an 07 range rover sport with the top end stereo but i have not listened to it yet (pick up car on sat) however i do like to listen to music loudly when doing my weekly 100 mile commute and as I have that huge boot I thought I could do with a removable sub box or something? now i am far from a "rude boy" i just enjoy crisp good sound with a nice deep baseline. Can anyone advise me of what i should / shouldnt buy? or maybe the current stereo will be enough. Thanks