Sub or No Sub

the short version of my setup (long version linked below)

PS3/PC into Logitech Z5500 system with B&W fronts and centre, small (8ft square) room.

I've been trying in vain to save up for a new sub/receiver in order to take the Logitech kit out of the setup but funds are hard to come by at the mo and I might, just, be able to splash £199 on a Sony STRDH820 when payday lands on Friday.

I know I could run the Logitech subwoofer on its own with a feed from the Sony but I was hoping to free it up to connect to my new(ish) TV.

Having had this unsofisitcated thug of a sub under my desk these last few years I really dont know what to expect if I switch to a 5.0 setup.  Fingers crossed I'll be able to get a demo of a similar setup but I was hoping to get some pointers from the WHF faithful so that I dont go into it blind so to speak.