in general you will be best served with the shortest distances between components, whether it be amps and speakers or your video equipment and display, etc. If you had to choose between having a larger distance between your surround processor and amp or the amp and the speakers, most people say let it be the former (processor/amp) because the interconnect cables have more shielding, while speaker cables have less. I have 15ft of speaker cable to the mains and it seems just fine.
as to the sw12. you are right. it has its own amp. I forget if youre using a sub now or not. basically, when youre not using a sub, all the frequencies are going to your 7bs, so the amp5 has to deal with the entire frequency range. if/when you add a subwoofer you will likely get the best movie sound by designating the 7bs as "small" and sending the lower frequencies (most processors designate 80hz as this point) to the subwoofer, therefore the subwoofer handles these frequencies and the 7bs- as well as the amp5- wont be handling these frequencies. in essence you are using 2 amps to drive the same frequency range. you might find that youll get better music sound this way. additionally, with the sw12 there are so many excellent controls on it, that Id suggest instead of using the processors dedicated subwoofer out, that you at least experiment telling the processor the 7bs are large speakers, take the right and left output into the sw12 and use the sw12s xover. adding a sub is a bit of a painstaking endeavor because you have to deal with speaker placement and xovers. definately leave a nice amount of time for the first day and you might need a couple of weeks to live with different setups. ultimately, if you watch movies I think a sub would definately be worth it and I also think it would probably improve your music experience as well. there are many fine subs out there, to me the best commercial sub that "does it all" is the sw12- its expensive though.