In principle, the AV8801 won't have anything that will cause a significant difference in functionality or audio. However, it will have construction improvements that some people enjoy having.
For example, the SR7009 has power amplifiers built into it. Since it's a pre/pro, the AV8802 won't. I suspect that, like the AV8801, the 8802 will contain discrete HDAM preamp circuit boards in place of the power amps. The other major difference will be that the AV8802 will have both RCA and XLR preamp output connections, while the SR7009 has only RCA preamp connections (in addition to its speaker connections). The pre/pro doubtless will have a copper-plated chassis similar to the 8801's.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped