The LFE+Mains setting allows you to use a crossover point for the mains when they are set to large preventing the doubling of all the bass frequencies.
And that is exactly the anomaly that has been identified by Audioholics. In a Denon, when you set the mains to "Large" they get all frequencies, regardless of the xo point and regardless of the "LFE" or "LFE+Mains" setting. This would cause the doubling of bass that you hear.
Now, you could get around it by setting the xo on the sub rather than the AVR, but in doing so, you are basically making them "Small" speakers anyway.
With a Denon, the correct setting is "LFE+Mains", all speakers set small.
