The ULF Trim boosts the deep bass output. The deeper the frequency it is set to, the more of a boost the deep bass gets. If you like to listen to movies at loud volumes, it might be a good idea to keep this higher, closer to 50 Hz.
The crossover switch will let the ULS-15 run full range if it is set to 'out'. If it is set to 'out', it is up to the AVR to filter out high frequencies. I would set it to 'out' and let the Onkyo deal with the crossover.