1. If I connect using LOC most on the market are for car audio and says max RMS 50W. Is that a concern.Jensen Iso-Max SP-2SX is your answer, it use transformer isolated the speaker output to xlr output up to 750W! And you don’t have to worry any hum introduce to the system or upgrade to a balanced power amp.
https://www.parts-express.com/jensen-iso-max-sp-2sx-2-channel-speakon-speaker-level-to-xlr--line-lev...
2. Once I successfully connect the subs using LOC and adjust the crossover on the sub - can I have another XLR connection from my Marantz AV to use as LFE for movies? Will dual setup work?Check with your subwoofer manufacturer, most subwoofer in the market don’t accept both connection simultaneously, you can use a ARX RS-1 switcher switch between 2 channel from DAC1/LOC and Marantz AV7005 sub out.
https://www.markertek.com/product/arx-rs-1/arx-rs-1-remote-switcher-stereo-xlr-balanced-remote-contr...