I'd suggest a receiver that includes Audyssey equalization. That'd probably make the most noticeable improvement. Assuming the old receiver has preamp inputs and the new one has preamp outputs, you might consider keeping the current one and splitting the speaker load between them.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped