Does that mean that's all av processors do? i.e. just cut everything else by -10dB
I believe a receiver will adjust all channels by -10dB, perform bass management, and then raise all channels again by 10 dB. The subwoofer output then has an extra opamp in the signal path that increases the subwoofer level by 10 dB.
When you are using Room Correction to do bass management, JRiver essentially does this and expects you to use your sub's amp to increase its level by 10dB to to manually do it in JRiver. The external increase in the sub's amp increases signal levels for other channels. You could probably do something similar in JRiver by using Room Correction before Convolution and then Parametric Equalizer after Convolution
Room Correction - +10 dB to LFE by actually adjusting all other channels by -10dB
Convolution - mix LFE and rerouted bass
Parametric Equalizer - Add Adjust the Volume and increase all channels except subwoofer by 10 dB
Subwoofer - increase volume level
You can check that it is working properly using Tools > Advanced Tools > Audio Calibration > Mode:Volume Calibration
Finally, just to double check, this means doing nothing (no delays, no XO, no nothing) in Room Correction except adding 10dB to the sub channel which at this point in time is the LFE track only because bass mgmt is happening in the subsequent filter.
Yes, you don't need to do anything else in Room Correction other than activate it and add 10dB to the sub channel.