As for integrating subwoofers, I suggest you read up on the Vandersteen 2Wq subwoofer at the Vandersteen site. IMHO, this is the easiest sub to integrate with any mains that extend down to 40Hz.
As for the demo issue, it's a tough one. For me, personally, it is no contest. I find SF speakers much better sounding than B&W speakers, and I have heard the newest 803 several times. But, speakers are like food, there is no one "best" for everyone. Like you said, though, I can't imagine you being dissappointed with the SFs. Buyer's remorse is a difficult thing. There are many examples of a person buying what they thought was the best speaker, DAC, amp or whatever that they could afford, only to discover something better, and maybe even cheaper, shortly after. The key is getting the best performance out of the gear you choose, and just enjoying listening to music.