I'd like to expand my 6.1 system to 7.1, for which I would purchase a pair of dipoles for the sides to compliment the monopole rears. The choices, to match my fronts, would be Mordaunt-Short 903S bipoles or 506 THX dipoles. The delima is that while the 903S' cost less than half as much and are smaller, they are only rated down to 120Hz. The THX versions are larger and more costly, but go down to 80Hz. If I get a receiver with a fixed crossover of 80Hz, will that leave too much of a bass 'hole' in the rear if I buy the 120Hz speakers? The rear centres will be MS 902 monopoles, which can go down to 55Hz. I'd rather not get a subwoofer for the rear at this point.