Mike, welcome. So far as the crossover for the mains, the fact that they may have good response to 40Hz or whatever doesn't make it necessary(or even advisable)to cross them at lower than 80Hz(whatever crossover setting you select on your 5790 will apply to all speakers set "small"). If you have a good sub it should be allowed to handle what it does best in the low bass and relieve the mains(and the 5790's amps)of part of the low bass burden. Another point, although it isn't entirely clear from your post, if you think that using the internal sub crossover affects the other speakers, that isn't the case; it's simply a low-pass for the sub itself.
