It is typical for receivers' and processors' multichannel analog inputs to act as an analog bypass - which in turn means that the bass management (which is done in the digital domain) does absolutely nothing for that input. In other words, it makes sense that your Denon 1907 isn't giving your FD51 any bass management via multichannel analog.

This issue is exactly why the Model 950 (and the Model 1070/970 as well) has a toggle switch on the rear panel. The toggle controls an 80Hz analog bass management system specifically for the multichannel analog input. There's no ability to control crossover points or manage which speakers are large or small (they're either all large or all small, and the small crossover is 80Hz).

The Model 1070/970 does offer the option of converting the multichannel analog input to digital so the processor's standard digital bass management gets applied. The Model 990 also does this, as the 7.1 analog input is converted to digital and piped through the bass management unless all speakers are set to "large" in the 990's setup menu.

If you wanted to be able to control crossover points on a channel-set basis (fronts, center, surrounds) while remaining in the analog domain, you would need Outlaw's old ICBM-1.