Unfortunately, the question as to whether or not modern, high quality, multichannel pre/pros can handle two channel audio as well as a dedicated preamp almost invariably starts flamewars. My personal takeaway from that is that in many cases they're too close to call.
Automated room EQ often can compensate for infelicities in speakers and room acoustics for two channel as well as multichannel surround sound. A failing(?) of the McIntosh MX121 is that it includes only Audyssey MultEQ XT, while the Marantz AV8801 has the much superior Audyssey MultEQ XT32. Of course, a well treated room with quality speakers often can reduce the need for electronic EQ.
The Marantz AV7702 pre/pro will include SubEQ (as part of Audyssey MultEQ XT32) so it will be comparable to the AV8801 in that regard, but it won't be available for several months. (Note that SubEQ is the name of Audyssey's feature which supports separate distance and sound level settings for two separate subwoofers. Audyssey MultEQ XT can EQ a single subwoofer without that.)
The other major feature of the AV7702 (and the AV8802 in February) will be Dolby Atmos. Supposedly Atmos soundtracks will be available on some BD and streamed titles by the holidays.
Both features (XT32 & Atmos) will be included in the Marantz SR7009 receiver, which is expected to be available in a couple of weeks, with a list price of about $2K. It will have preamp outputs but unfortunately won't have XLR connections.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped