I would recommend caution. Very few people use a sub with your speakers. I'm not saying it can't be done, just that there's a high potential for mistakes. If I was in your position, I would contact John at Wellington Audio in FL. http://www.wellingtonaudio.com/
He's an expert on British audio products. I can't think of anyone more competent to help you. If you live in CA, maybe try Gene Rubin if he's within driving distance. Other than that, I would call John. I haven't been to his store in a few years, but last time I was there we just happened to be talking about subs for small British speakers like you have. He's one of the most honest people in the industry, and will not try and sell you something that's not in your best interest.
He's an expert on British audio products. I can't think of anyone more competent to help you. If you live in CA, maybe try Gene Rubin if he's within driving distance. Other than that, I would call John. I haven't been to his store in a few years, but last time I was there we just happened to be talking about subs for small British speakers like you have. He's one of the most honest people in the industry, and will not try and sell you something that's not in your best interest.