Your description does suggest that your receiver is unable to provide enough power to drive your speakers to appropriate volume levels.
If you haven't already done so, make sure the main, center and surround speakers are all set to Small in the receiver's Speaker Setup and that the crossover frequency is set to 80 Hz or higher. This will divert the power-hungry low frequencies from the speakers to the powered subwoofer, offloading the amplifiers in the receiver.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped