In need of some advice on a matching a sub

Hi guys new to all this just wondering if anyone could help me out, im currently running a 5.1 (without sub) system im using kef reference model 4s for fronts model 1s for rears and a 200c center speaker im running all this off a avc a1-hd intergrated amp, ive done some reasearch and been told to pick a sub very wisely as it will make or break the system just wondering if you have any suggestions thanks