While it's still possible to see reggae legends like Lee Perry and Horace Andy at Bristol's large venues (ie Colston Hall and the o2 Academy), live reggae in Bristol is fewer and farther between. Dub and 'soundsystem culture' on the other hand, are literally huge. For the big dub and roots soundclashes, keep an eye on whats on at the Trinity Centre in Bristol, most notably Unity in Dub who's guest soundsystems have included Irration Steppas and the mighty King Earthquake.
On a much smaller scale look at Cosies listings for their weekly roots and reggae night and check whats on at the Attic (attached to the Full Moon on Stokes Croft). Pure ragga, bashment or dancehall events in Bristol are pretty rare, instead a reggae room is a pretty common fixture at big nights at venues like Lakota.
The rapid growth of dubstep in Bristol has brought with it a rekindled interest in modern dub (aka digi-dub aka future dub) and has led to some unlikely dub bookings like Mad Professor at Shit the Bed in the Motion listings.