i played with ypao briefly on my 5790. i was impressed that it *perfectly* calculated speaker distance, but it did some other things that i wasn't so sure of. i haven't done extensive goofing with it, so i can't give a definitive response.
i'll just say it seemed to work pretty well, and for someone who doesn't want to tweek-out, it's a great alternative.
regarding the 5790 vs. the 1400, according to yamaha, they're the same. so you may as well get the cheaper one. check out
this thread - the last question.