The crossover control on the back of the T/5i only affects the High Level and Low Level inputs, and does not alter the signal coming in through the .1/LFE input since the crossover for this channel is set by your AV processor. For home theater applications we recommend using both the High Level input (connected to your Marantz MM7025) and the .1/LFE input (connected to the AV7702 AV processor), which means the subwoofer will provide low-frequency extension for your SF Cremonas and at the same time play the LFE channel for surround sound material.
We recommend setting the front left and right channels to large, then setting the T/5i’s crossover control below where the Cremonas are naturally rolling off in their low-frequency extension. This is most easily tuned when listening to music with rhythmic and deep bass, rather than using a movie. After tuning the High Level input disconnect the Speakon cable from the back of the sub, leaving only the .1/LFE input connected, and run the room correction. After the room correction is finished you can reconnect the High Level cable and use the system with both connected. We have a good blog post on our website which details this and hopefully makes things clearer: https://rel.net/how-to-use/
Regarding the phase setting, you are absolutely correct regarding what to listen for, but we have noticed that in some rooms and systems switching the phase control back and forth does not make an audible difference. In this case you can leave the switch in the zero degree position.