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CO is the only other one-connection routing. They only run ERJs on the BNA-EWR route, so that is something you'll have to suffer through for a couple hours to get up to EWR for the onward flight. They operate the EWR-MAN route with the 757-200s. Right now ~13/41 of the 752s have AVOD throughout the plane, as well as power at all seats. The non-AVOD planes have emPower plugs in the front half of the plane (exit rows to the front, inclusive of the exit rows). CO is continuing to upgrade their 752s and expects to have the entire fleet converted by the end of the year. I do not know the schedule for DL's conversions.

All the other options are double-connects, which I don't think is worthwhile.

Also, if you are buying cheaper fares the CO flights will only earn you 50% MQMs for your DL elite status, which would likely prevent you from making PM and getting free lounge access in ATL/EWR on these trips.