Why 720p? Is that what your TV supports?
I would go with 1080p on all boxes. You have nothing to lose by trying.
Just got xfinity TV installed after having directv for several years. I have an Xg1v4 (HD DVR) and XiD-P (HD, non-DVR), both are set to output 720p, both hardwired. I recorded a show on HBO last night and played it back on the XiD-P. The picture quality was pretty terrible. Dark scenes were a blurry mess, looked like super low bitrate, worse than it looks watching it live on the same TV/box. I went to the xg1v4 and played back the same recording and it looked noticeably better, equivalent to the live broadcast. Is there something wrong with my setup or equipment or is this normal behavior with recordings on non-dvr boxes (I hope not)? With directv I had a similar setup using the same outlets/wiring/TV’s/HDMI cables and the video quality of recordings was equivalent no matter where it was watched.
Satellite and cable are different. You might need to upgrade your equipment.
adding... did you record an SD or HD HBO channel? (not all are HD)