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I think this would be due to the delay on the rear speakers, if the sub is a dedicated rear sub from the processor. Maybe you could play with the delays and see if that homogenizes the sound more.

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I'm not sure about the specs on the NHT's, but if the rears are small and that's why you added the rear sub, you could still get some bass management on that channel by using line level inputs to the sub and the outputs to the rears, assuming the NHT sub has these features. If the sub is to function as a rear sub for 3 channels instead of two, this scheme would not work satisfactorily.

A setup question I have is are you using the ICBM between the McIntosh and Parasound, or at some other position? My only concern was if you would get the cascading crossover problem if there is bass management included in the other processors. I could be totally off base, but have been bitten by that before.

S.