do you have a surround sound system ?
only asking because, if you do, then you could just connect up your sky/freeview etc! to the surround amp and have surround sound + BASS
however, if you dont have surround sound, you can connect a sub to the TV with a standard sub cable (phono - phono) to add a little more bass as most plasmas/lcds have pretty weedy built-in speakers (with some exceptions).
what sort of budget were you looking at for a new sub ? & have you considered buying a surround sound setup ?
something like this yamaha setup
http://www.petertyson.co.uk/ebuttonz/ebz_product_pages/yamahaav59-yamahaav59.shtml?dtcpc .. for around £230 you get 5 satellite speakers, a subwoofer and an AV amp .. that you connect up pretty much anything to .. sky, games consoles, vcr, dvd player etc! .. and also reap the benefits of dolby digital 5.1 & DTS
paul