The Geddes Approach: No Conventional Crossover
This is the strategy recommended by Earl Geddes for use with his speakers. In this approach, the AVR or pre-pro is set to the "LFE + Main", "double bass", "plus" or similarly-named mode, depending on its manufacturer. The main speakers are run full range, and the AVR crossover frequency is set to the highest allowable value, often 250 Hz. A separate low-pass filter for each sub is configured in the DSP device connected to the AVR's sub out, but only during and after optimization. Prior to measurements, there are no LPFs or other filters configured in the DSP. Instead, the DSP is configured to have all its filters bypassed. These low-pass filters could in general have a different cutoff frequency for each sub, and the cutoff frequency is chosen so the frequency ranges of subs and mains overlap. Filters of the desired type are added in MSO, which will then determine the optimum cutoff frequency for each one. It should be noted that the Geddes loudspeakers are of closed-box type, with high power handling and an anechoic low-frequency cutoff of 80 - 90 Hz. If your mains are of the vented-box type, you'll still need to high-pass filter them, which means that you need user-configurable DSP on the main speakers as well. AVRs or preamp-processors that provide such capability are rare.
In the past, Earl Geddes has recommended that dissimilar subwoofers be used, with one high-output sub, and two or three additional subs having lesser output. MSO assumes linear operation, so running an optimization without taking maximum output into consideration could lead to a solution that is sub-optimal with respect to maximum output capability. Since this potential problem is not limited to the Geddes-style configuration, it is discussed separately on the Working With Dissimilar Subwoofers page.