I would definitely skip the Polk if you want good bass. I would lean towards SVS. It is definitely the case that as you pay more for subwoofers you get more. In my experience, a really expensive subwoofer from a reputable manufacturer gives a ton more bass response then their cheaper models, so if you want to really feel (and not just hear boomy sound), then spending more makes sense.
As for speakers, I am rarely a fan of models which can cost more, but use the same size drivers as some other company with a much less expensive model. So, it may be worth listening to competing products with similar 5" drivers (or larger) and see how they sound. But, if you've listened to the Wharfdales and like how they sound and it fits your budget, then go for it.