I've heard that there are some online sub makers out there e.g. AV123, HSU, eV, etc...

I want to spend about $400-$600 on a sub for use with B&W speakers but am not sure which to go with. Usually I buy speakers from a store and 'test' them. If I don't like them I take them back.

How can I therefore be sure to pick a good sub when I am told that most quality sub builders only sell online?

Thanks.
Seth.

Epik Knight, mwf-15
There is a lot of first hand impressions on the former and they're all pretty far from being disappointed.
The mwf-15 was scored phenomenally high by Craig, assuming it was scored fairly it's the best option for your budget but there's gonna be a really long wait on it.
I would go with a Knight just cause you can get it almost immediatly.
HSU doesn't have anything comparable in the price range.

At the lowend of your budget Cadence cx-15 15" subwoofer for $400 shipped.

At the highend outlaw LFM-Ex $600 shipped.

How can I therefore be sure to pick a good sub when I am told that most quality sub builders only sell online?
You start by sifting through the multitude of comments here,audioholics, and avreview to find as much information on the subs you are interested in. Then visit the ID's sites, SVsound, hsuresearch, etc....
Most, if not all of these companies will allow you a trial period during which you can return the sub. You will probably have to eat shipping costs which can be high due to the size and weight of the subs.
Once you narrow down your choices, ask around avsforum for owners of the equipment you are interested in who live in your area and you will find many fanatics who would be happy to allow you come by and listen for yourself.