u/Clean-Dirt4160

I am seeking technical advice for my 5.1 home theater setup. I am using a Denon AVR-1312 [connected with an Optical Wire to the tv] with customized JBL rear speakers and a Xiaomi MiTV front array. While the front stage sounds excellent, the rear speakers—mounted at the top rear corners—are producing very low volume and poor sound quality. I currently have the AVR set to Dolby Pro Logic II Cinema. Given the narrow dimensions of my room (17.5' x 6.5'), could you please help me calibrate the levels?