Hmmm, -18 dB is where I watch just about everything. I don't ever notice what seems like extra bass (I don't use the Tone control at all), actually without the Loudness things seem a little anemic in the bass range. Have you run the MCAAC? With 3-position standing wave attenuation? I think that would still have a little effect even with 2 channels, because it does make adjustments for the main (center, and sub too, if you have them).
Though as I'm always quick to say, what ever sounds best to you.
