AB,regardless of the crossover flexibility that a particular receiver may offer, there may be little reason to set a crossover lower than 80Hz on any speaker. THX and others found that to be a good compromise, being low enough so that the sub generally can't be easily localized and being high enough to take a significant part of the lowest bass burden off the mains(and the amps driving them)and putting it onto the sub, where it belongs.
Be that as it may, you really don't have to make the choice that you discuss; the Onkyo 703 has individual crossover settings for the different groups of speakers, similar to the provision in the HK models.
